Harris and Trump face major challenges, risks on Tuesday's presidential debate stage

Vice President Kamala Harris and former President Donald Trump will need to navigate the pitfall-filled debate of their political lives on Tuesday as each tries to persuade millions of voters and viewers that they're the one best suited to be president.

Harris, whose wave of momentum has brought Democrats back to a neck-and-neck presidential race, will have to prosecute the case against Trump while also laying out how her agenda could help the country -- particularly beleaguered middle- and working-class Americans.

Trump, meanwhile, has the task of casting his record on the economy and immigration as superior to Harris' while avoiding distracting personal attacks on Harris.

The ABC News presidential debate will take place on Sept. 10 at 9 p.m. ET and air on ABC and stream on ABC News Live, Disney+ and Hulu.

The showdown in Philadelphia is taking place months after the last debate ended President Joe Biden's reelection bid, sending Harris rushing to stand up an eleventh-hour campaign and Trump scrambling to figure out how to negatively define a new opponent voters are less familiar with.

"I think there's an outsized expectation of 'gosh, the last guy dropped out, let's watch it.' So, I think that there's a lot more at stake than normally I would ever say is at stake," said Sean Spicer, Trump's first White House press secretary.

"I'm not a huge believer that debates move the needle that much," he added, "but I do think that because of the nontraditional nature of what's happening right now, there's going to be an outsized degree of attention."

Democrats who spoke to ABC News said that Harris has two main goals: affirm to voters that she is ready to lead the country and the free world and to describe in more detail what policies she'd pursue as president.

Some Democratic sources said that voters could be concerned by Harris' rapid ascension as the Democrats' nominee and -- unjustly, they said -- her gender when thinking of the kind of president they'd feel comfortable with.

A strong debate performance could allay worries and cement the momentum she's enjoyed to date.

"I think she has to answer the overarching question, which is, can she lead the country, and what type of president will President Harris be? People just want to be comfortable in that decision," said Bakari Sellers, a prominent Harris ally. "I don't want her to be timid at all. Just be yourself, be comfortable, answer questions and turn around and hammer him."

Trump has sought to cast Harris as "dangerous" by painting her as a "California liberal" who was soft on crime and generally out of step, referencing her time as state attorney general and senator and relying on voters' perceptions of the progressive bastion to fill in the blanks.

That's something Harris could use the debate stage to push back on, possibly repeating parts of her stump speech in which she details her efforts as California attorney general combating transnational gangs operating across the southern border.

One source familiar with the Harris campaign's thinking said the vice president should "clearly [stake] out where she stands on issues like the border and crime and [talk] about her record on those issues as a prosecutor and attorney general to demonstrate that the portrayals are misrepresenting her actual views on those issues."

That defense will likely be complemented by an effort to highlight Harris' economic policies, which she's recently begun to roll out, to also address voter worries about inflation.

Harris has introduced plans to make it easier to buy a house and start a small business, while, in a nod to the business community, saying she'd also increase the capital gains tax by less than Biden has proposed.

"It's an important opportunity for her to continue to lay out her economic vision, to demonstrate both through talking about her experience and her vision, that she will be a strong leader, as she has said, for all Americans," one source close to Harris' team said.

However, Harris is facing off against maybe the most unpredictable non-traditional figures in modern politics, and Trump is likely to throw in curveballs that could take the vice president off her talking points.

Trump has already launched a fusillade of personal attacks, including questioning Harris' race and intelligence and highlighting vulgar suggestions about sexual acts.

So far, Harris has barely responded, casting the barbs as the "same old, tired playbook."

Now, some allies would like to see her fight back.

"I think there is a mechanism whereby you stand up to bullies and you call it out for what it is and simply say that, 'while the former president is using racism as political currency, I represent a new future, one where we don't divide people and use such degrading terms to anyone,' Sellers said. "I would look him dead in the eye and say, 'former President Trump, we are better than you right now.'"

Others weren't so sure.

"I would ignore what are likely to be rude, disrespectful behaviors from Trump, and stay focused on the substance, because by doing so, it will further highlight for people just how disgusting his behavior can be," said the source close to Harris' team.

Republicans, for their part, hope to avoid that scenario altogether.

GOP operatives who spoke to ABC News said Trump should focus on policy contrasts, boasting that he has the edge on issues like inflation and immigration and can try to pin her down on her policy reversals on things like fracking – while he himself searches for consistent stances on issues like abortion.

"He's not going to have many other windows where Kamala Harris is going to be asked tough questions and tough follow up questions, and so he needs to keep his responses and very focused on her issue positions that have come out of her mouth and make her reconcile what she's saying now with what she said in 2020," said GOP strategist Brad Todd.

"When she says, 'I'm not for banning fracking,' then he needs to say, "so, you wrong before? What caused you to believe that your previous position was wrong? Or are you just worried about Pennsylvania?'" Todd said, referencing the swing state's economic reliance on the practice.

Trump has at times knocked leaned into that message, knocking her promises for "day one" by noting she's already been in office for almost four years serving a president facing severe disapproval ratings when he dropped out of the race.

It's unclear precisely how effective that tie could be -- an ABC News/Washington Post poll last month showed that only 11% of voters said Harris had a great deal of influence over economic policy, and just 15% said the same of immigration policy. But Republicans urged Trump to hammer the connection.

"For him to be viewed as having a successful debate, he has to continue that assault," said one former campaign aide in touch with Trump's current team. "She's the vice president United States seeking the second term of Joe Biden. We can make that case."

Still, Trump has a proven penchant for veering off into unrelated attacks, whether it be against opponents or moderators -- a strategy that has helped him on the stump but one that could backfire on Tuesday.

"If he takes the bait and makes some kind of one-off comment about her and calls her names, I think that's going to be the story the next day," Spicer said.

"He's a field player and he's an improviser, and that's what's made him effective as a communicator," Todd added. "But this is a time for discipline."