The Trump-era opinion that might stymie the Biden impeachment inquiry

A Trump-era opinion issued by the Justice Department Office of Legal Counsel is being highlighted as a possible barrier for House Republicans in their impeachment inquiry against President Joe Biden.

Speaker Kevin McCarthy on Tuesday directed House committees to move ahead with the inquiry without a House vote. A lack of support among his caucus made it unclear whether he would've had the votes to launch the probe.

Now, the Biden administration could focus one part of their legal defense on a 2020 DOJ opinion that took the position that impeachment investigations by the House are invalid without a vote of the full chamber.

"The Constitution vests the 'sole Power of Impeachment' in the House of Representatives ... For precisely that reason, the House itself must authorize an impeachment inquiry, as it has done in virtually every prior impeachment investigation in our Nation's history, including every one involving a President," Steve Engel, a top lawyer for the department, wrote in the opinion.

Engel added, "no committee may undertake the momentous move from legislative oversight to impeachment without a delegation by the full House of such authority."

A Justice Department spokesperson didn't immediately respond to ABC News' request for comment on the opinion. Politico first reported on the 2020 opinion after McCarthy's announcement Tuesday.

White House counsel spokesperson Ian Sams was asked if the administration would use the opinion as a guiding force in their response during an appearance on CNN on Wednesday morning.

"I think we're going to see what they're going to do," he responded. "I don't want to speculate on what the House Republicans may or may not ask for. I mean, they won't even vote for it and they can't even say what they're impeaching him for. So I think we're going to have to see what exactly what they try to do with this newfound impeachment inquiry."

The opinion was written in response to then-House Speaker Nancy Pelosi's launch of an impeachment inquiry against Trump in 2019 over allegations the president abused his power by pressuring a foreign official to investigate Biden.

Pelosi announced the inquiry in September 2019 without a full House vote. A resolution formalizing the inquiry was later approved by the full House in late October.

McCarthy had previously said a House vote would be taken before launching an inquiry, telling Brietbart in August an impeachment probe would "occur through a vote on the floor of the People’s House and not through a declaration by one person."

Asked about his apparent reversal, McCarthy told reporters Wednesday he "never changed my position" and pointed to Pelosi's past action.

“Nancy Pelosi changed the precedent of this House on September 24," he said.

Top House Democrats pushed back on McCarthy's unilateral maneuver during a press conference on Wednesday morning.

Democratic Rep. Ted Lieu, who served as an impeachment manager for Trump's second impeachment, called it "impeachment theater" and claimed the 2020 DOJ opinion means House Republicans will be granted no additional power until a vote is held.

"There is no additional power the Republican Congress has simply because they called this impeachment inquiry and that's because the Department of Justice under the Trump administration issued a ruling stating unless there is a vote of the House, there is no additional power of the House for doing an impeachment inquiry," Lieu said.

While such opinions don't dictate how a court would eventually rule on a potential legal challenge brought by House Republicans should the Biden administration resist the probe, the opinion is currently considered binding between administrations unless Attorney General Merrick Garland's Office of Legal Counsel issues a new opinion altering their approach to such inquiries.

-ABC News' Alexandra Hutzler contributed to this report.