Key Senate races that could tilt the balance of power toward Democrats on Election Day

As President Donald Trump and former Vice President Joe Biden battle for the presidency in November, Republicans and Democrats are wrestling over the Senate -- with a handful of races likely to determine which party will control the chamber in 2021, and with it, influence over the next president’s agenda, Cabinet and judicial nominees.

With Republicans holding a 53-47 majority, Democrats need to win back at least four seats -- three, if they also win the White House since the vice president breaks 50-50 ties -- for control of the Senate. They will also have to grapple with the likely loss of Democratic Sen. Doug Jones in Alabama as they try to flip the chamber.

As Biden consistently leads in the polls, and the president’s handling of the coronavirus pandemic sinks his popularity, the number of competitive races across the map has expanded into traditionally red territory like Texas, Georgia and even South Carolina, where Democrats and aligned outside groups have also outspent their GOP opponents thanks to record-breaking fundraising hauls.

The outlook is looking increasingly bleak for Republicans -- so much so that even Trump admitted it will be "very tough" to keep the Senate, according to a Washington Post report about his remarks at a private fundraiser last week.

Of the dozen races that are competitive in 2020 for either party, here is a look at six key Senate races this year:

Democrats hope for a 2018 repeat in Arizona

While Trump won Arizona in 2016 against Hillary Clinton by roughly 90,000 votes, Democratic Rep. Kyrsten Sinema’s victory over Rep. Martha McSally in the 2018 Senate race gave Democrats confidence that the rapidly changing Sunbelt state could be in play in 2020 on the presidential level, too.

And McSally, who was appointed to fill the seat held by the late Sen. John McCain in December 2018, is in danger of losing a second Senate race to Mark Kelly, a former Navy captain and NASA astronaut who became a leading gun-control activist after his wife, former Rep. Gabby Giffords, was shot in the head and nearly killed by a gunman while meeting with constituents in 2011.

Kelly, who is running as a moderate, has led McSally in nearly every public poll of the race in the final months of the election, a more consistent lead than Biden’s slimmer margin over Trump in the state.

For the second consecutive cycle, McSally is tying her political fortunes to Trump, hoping that the support of his base is enough to offset the movement of Latinos, women and suburban voters toward Democrats.

It’s led to a difficult balancing act underscored in a response she gave when asked at an Oct. 6 debate if she was proud of her support for Trump.

"I'm proud that I'm fighting for Arizonans on things like cutting your taxes," she said, without mentioning the president. "I'm proud to be fighting for Arizona every single day.”

Democrats confident in Colorado

First elected to the Senate in 2014 on a wave of backlash to the Obama administration’s handling of the Ebola virus and the ascent of ISIS, Sen. Cory Gardner is in a similar position as McSally: running for reelection in a state where the president is deeply unpopular.

While Arizona is in the midst of a potential political realignment, Colorado has already made the shift. No Republican presidential candidate has won the state since 2004, and the Trump campaign has abandoned its early vows to compete here.

Gardner has worked to cultivate his own brand to distinguish himself from the president in the minds of independent voters. He led a bipartisan effort to secure funding to address the backlog of maintenance issues at national parks - a law repeatedly touted in his campaign ads.

But it’s unlikely that his efforts will be enough to keep his Senate seat in a state Trump is expected to lose handily, even as his opponent, former Gov. John Hickenlooper -- who entered the race after his long-shot White House bid fizzled -- has faced criticism for violating state ethics rules by accepting a jet flight and other benefits he didn’t pay for on a trip to Italy. (Hickenlooper paid a $2,750 fine and acknowledged wrongdoing.)

“This isn’t a question of pride, this is a question of getting through this together,” Gardner said when asked in an Oct. 9 debate whether he was proud of Trump’s coronavirus response. “I believe we must get through this by staying together, staying united.”

The last New England Republican?

Sen. Susan Collins, R-Maine, helped tank Trump’s plans to repeal and replace the Affordable Care Act by voting against the GOP health care proposal in 2017 and voted to hear witnesses in the president’s Senate impeachment trial. She also said she couldn’t support the president’s campaign four years ago.

Republicans argue that her record over the last four years has added up to boost her independent credentials, but Democrats have seized on the pro-choice senator’s votes for Supreme Court Justice Brett Kavanaugh and to acquit the president in his impeachment trial to cast her as an enabler of the president’s agenda, despite her occasional criticisms of his words and actions.

Collins, the only Republican holding federal office in New England, has deep institutional support in Maine -- most notably from the Bush clan -- but the state, like many other battlegrounds, has trended increasingly blue since 2016, when Trump nabbed an electoral vote by winning Maine’s 2nd Congressional District.

Maine House Speaker Sara Gideon, the Democratic candidate, has repeatedly outraised Collins, and public polling has shown Gideon ahead of the four-term senator.

Could the GOP lose a seat in the Hawkeye State?

It's a state that Trump won by 10 points in 2016, and senior Sen. Chuck Grassley won reelection by over 20 points the same year.

Four years later, Sen. Joni Ernst, who was first elected in 2014 when she catapulted onto the national stage with a promise to "make Washington squeal," is now fighting for her political life. In a race with Democrat Theresa Greenfield, a businesswoman, recent polling shows the two women neck-and-neck.

Ernst has remained loyal to Trump, sticking by him even as other Republicans put distance between them and the president and as his polling in the state slumps. But as the president currently trails former Vice President Joe Biden in a New York Times/Siena College by a three-point margin, her campaign tells ABC News that the senator has been running her own campaign and is willing to work with both Democrats and Republicans to deliver for Iowa.

Heading into the final stretch, Ernst has sought to cast Greenfield as out of step with Iowa voters, accusing her of representing the views of voters on the coasts, rather than in rural America, on issues like abortion.

But Ernst recently struggled over an issue that is critical to Iowa voters -- soybeans. Asked for the breakeven price for soybeans in Iowa twice, the senator first deflected, before responding, "probably about $5.50."

"Well, you're a couple dollars off I think here because it's $10.05," the moderator said.

In addition to Ernst’s stumbling over a question about a product that is crucial to Iowa's economy, Greenfield’s significant edge in the money race -- bringing in $21 million more than Ernst in their most recent filings -- has worried Republicans who once considered Ernst one of the safer incumbents earlier this year.

One week out, Greenfield, who describes herself as a "scrappy farm kid," is looking to make gains in rural areas, win over those counties that voted both for Trump and former President Barack Obama and run up the score in suburban Iowa.

The Senate could hinge on North Carolina's Senate race

In the all-important Tar Heel state that is crucial to Trump's pathway to reelection, Sen. Thom Tillis is in a tight race for a second term against Cal Cunningham, a former Democratic state lawmaker and Army veteran.

Tillis was widely seen as endangered from the race's onset -- a Democratic strategist told ABC News last year that Tillis is a top target for Democrats in 2020, who see him as "one of the weakest incumbents." An ABC News/Washington Post poll released last week shows Cunningham with a slight edge, 49% to 47%.

Tillis, whose electoral fortune is likely tied to Trump's, is navigating a delicate balancing act between running his own race without alienating Trump's base. He even pointed out the importance of keeping the Senate in GOP hands in the event that Trump loses, recently telling Politico the “best check on a Biden presidency is for Republicans to have a majority in the Senate. And I do think 'checks and balances' does resonate with North Carolina voters."

But a number of late-breaking episodes have reverberated in North Carolina, including Tillis’ battle with COVID-19 in early October, and text messages revealing Cunningham’s extramarital affair with a woman who claimed they had at least one intimate encounter. (The Army Reserves is investigating the allegations about Cunningham, who is a lieutenant colonel in the Army Reserves. Cunningham has confirmed sending romantic text messages to a woman who is not his wife, but he has neither confirmed nor denied the alleged encounter.)

Cunningham has kept a low profile in the final days of the race, using his appearances to focus on the issue of health care -- and the role Judge Amy Coney Barrett could play in determining the fate of the Affordable Care Act once seated on the Supreme Court.

Both candidates are hoping to capitalize on the coalitions forming around Trump and Biden: Cunningham is hoping to drive up turnout in the cities and suburbs, particularly among minorities and women, while Tillis is looking to the white enclaves in rural parts of the state and conservatives energized by the Supreme Court confirmation battle and the president’s repeated visits to the state. The race is essentially tied among independents and suburban voters, the ABC News/Washington Poll survey found.

Democrats don’t need North Carolina to capture the majority, but their path becomes harder without it.

Democrats on defense in Michigan

Sen. Gary Peters, a low-key incumbent who arrived in the Senate after winning in a wave year for Republicans in 2014, is seeking a second term in one of the most critical states for Democrats.

In his race against John James, a former Apache combat helicopter pilot in Iraq, Peters is largely spending his time highlighting his accomplishments for Michigan and tethering his opponent to Trump, who is behind Biden in a recent poll by eight points.

James, who lost a Senate bid two years ago, has tried to cast Peters as too liberal for the Rust Belt state won by Trump in 2016.

Despite Democrats’ chances of capturing Michigan in the presidential race, the Senate campaign has come into more focus, with recent polling showing a tighter contest -- and fundraising numbers -- than the presidential polling would suggest.

James has yet to publicly contrast himself with Trump or criticize him in any significant way -- in the hopes of shoring up support from Trump's base to capture a long-held Democratic seat. But he's also sought to retain independence from Trump.

"Look, I put my life on the line for the Constitution. Not any party or president. So when any president is doing right by Michigan, I'll help them. When they're not, I'll fight back," he said in a closing ad.

Michigan will certainly be a key barometer of Democrats' strength in the suburbs and among minorities in urban centers, with their hopes hinging on high turnout in Detroit and significant margins in its suburbs to rebuild their "blue wall." Before Trump carried the state by the thinnest of margins -- just under 11,000 votes -- the state was firmly in the Democrats' grip for two decades.

The wildcard races: South Carolina, Alaska, Georgia, Texas, Kansas

Biden’s surprisingly strong performance in Texas and Georgia has buoyed Democrats’ hopes in races to unseat Sens. John Cornyn of Texas, and Kelly Loeffler and David Perdue in Georgia.

In Texas, MJ Hegar has recently outraised Cornyn -- and wiped out his cash advantage too -- and could benefit from surging early turnout that has worried Republicans and potentially put the Lone Star State in play once again for Democrats, after Sen. Ted Cruz’s slim victory in 2018.

In Georgia, Raphael Warnock, the pastor of Ebenezer Baptist Church, the church of Martin Luther King Jr., is running ahead of Loeffler, who is fending off Rep. Doug Collins in the crowded race. Jon Ossoff, a former Democratic House candidate, is also in a neck-and-neck contest with Perdue.

Both races are likely to lead to runoffs with the top two finishers if no candidate secures at least 50% of the vote in November.

Trump-foe-turned-ally Sen. Lindsey Graham, R-S.C., is in a surprisingly close race with Jamie Harrison, the former South Carolina Democratic County Chair, who is banking on Black voter turnout, Trump’s unpopularity in suburbia and some Republicans’ wariness of Graham -- along with his record-breaking fundraising haul -- to deliver an upset in a state Trump is expected to win.

In a sign of Graham’s vulnerability, Vice President Mike Pence is scheduled to travel to South Carolina ahead of the election.

Alaska, where GOP Sen. Dan Sullivan is running against Dr. Al Gross, an independent aligned with Democrats, is another late-breaking state where Republicans have been put on the defensive.

Notoriously difficult to poll, its independent-minded voters and Gross’ fundraising advantage have left both parties preparing for a nail-biter.

And in Kansas, a state where Sen. Jerry Moran, won reelection by 30 points the last time Trump was on the ballot, an open seat vacated by Sen. Pat Roberts, has left Republicans on unsteady footing. GOP Rep. Roger Marshall is running against state Sen. Barbara Bollier, who boasts cross-party appeal as a Republican-turned-Democrat.

After Secretary of State Mike Pompeo, who would have been a heavy favorite to capture the seat, passed on a Senate bid, Bollier, a former doctor, is reaching for what once seemed impossible but is now a competitive race that only slightly favors Republicans.