Senators will determine not only the political fate of Donald Trump during the former president’s second impeachment trial next week but also whether or not to weaken their own congressional power to rein in presidential misconduct.

If that happens, it could undermine the reason the founders gave Congress the impeachment power in the first place: as one of the checks and balances in the Constitution to keep a president from becoming a tyrant, members of Congress, historians and constitutional scholars say.

“The fact that we can’t come together, both as a political body and as a nation, around the notion that an incumbent commander in chief cannot stage a coup against his own government in order to overturn the will of the people speaks to how far we have strayed from our ideals as a nation, without question,” said Mark Updegrove, a presidential historian for ABC News who is president and CEO of the LBJ Foundation.

Republicans have rallied around a position that the Senate doesn’t have jurisdiction to convict Trump on the charge of incitement of insurrection because he is no longer in office. Trump’s lawyers plan to lean heavily on that argument in his defense.

Such a conclusion would allow Republicans to avoid casting judgment on Trump, who remains popular among their party’s voters, for actions between the Nov. 3 election and the Jan. 6 insurrection, in which a mob of his supporters overran the U.S. Capitol.

But an acquittal for Trump on those procedural grounds would pave the way for the Senate to forfeit one of its important constitutional tools, said Josh Chafetz, a professor at Georgetown University Law Center who wrote a book about legislative authority and separation of powers.

“Impeachment post-officeholding is both the most powerful way Congress can express its disapproval of the way a former officeholder wielded office, and also, of course, a means of disqualifying for future officeholding,” Chafetz said.

Congressional power has already eroded in other ways, both before the Trump administration and during it. That includes weakened committee oversight power and questions about how much legislative power Congress should hand over to the executive branch.

A July Supreme Court ruling in a fight about Trump’s personal business and tax records, for example, for the first time put limits on congressional power to subpoena a sitting president’s personal and business information.

Unique power

On impeachment, the founders gave Congress the power as “a bridle in the hands of the legislative body upon the executive” branch, a Congressional Research Service report notes.

To bolster that power, the Constitution bars a president from using a pardon to shield individuals from impeachment. And the Supreme Court previously ruled it does not have the power to second-guess Congress on the impeachment process.

That means the Senate largely determines its own power, and Trump’s presidential impeachment trial will be just the fourth in history and the first for a president who is no longer in office.

Kentucky Republican Sen. Rand Paul already forced a procedural vote on whether to dismiss the impeachment ahead of a trial, which got the support of 45 Republican senators.

Had that succeeded, it would have eliminated another historic purpose of an impeachment trial, what Princeton University politics professor Keith Whittington dubbed “congressional oversight on steroids.”

“The impeachment power gives the first, most democratic branch of the government the ability to scrutinize the actions of individuals in the other branches of government and call them to account for their actions,” Whittington wrote last month on Lawfare.

Trump’s second impeachment trial will move forward because a majority of senators agreed to do so. But it takes two-thirds to convict, and enough Republicans appear poised to acquit Trump simply because his presidential term ended before the trial did.

That outcome would harm Congress’ ability to send a warning to future officeholders, Whittington wrote. If impeached officials can short-circuit that impeachment process by resigning or leaving office, “then the bad actor has it within his power to deprive Congress of the ability to fully make an example of him and send the necessary signals to future officeholders,” Whittington wrote.

The House impeachment managers plan to make similar arguments about congressional power during the trial.

“If the Senate does not try President Trump (and convict him) it risks declaring to all future Presidents that there will be no consequences, no accountability, indeed no Congressional response at all if they violate their Oath to ‘preserve, protect and defend the Constitution’ in their final weeks — and instead provoke lethal violence in a lawless effort to retain power,” the House impeachment managers argued in a brief filed Tuesday.

Procedural argument

Some Republican senators have argued that impeaching a former president would essentially give Congress too much power, as there is no limiting principle that would prohibit impeaching Barack Obama now if the political will were there.

“If the Senate were to adopt a broad interpretation of the impeachment power — one allowing federal officials to be convicted on impeachment charges even after leaving office — the result would not only be problematic, but also contrary to the most natural reading of the text, structure, and historical understanding of the Constitution,” Utah Republican Sen. Mike Lee said in a news release.

The House impeached Trump on Jan. 13, while he was still in office. But Senate Majority Leader Mitch McConnell said a trial would not happen before Trump left office on Jan. 20. The House did not send the impeachment article to the Senate until Jan. 25, five days after President Joe Biden’s inauguration.

Any impeachment hearing is always seen through the lens of politics at the time, and not necessarily the Constitution, Updegrove said.

“While senators opposing this trial are maybe doing so in the interest of political expedience, they will not sidestep the judgment of history,” Updegrove said. “This is as pernicious an act as a president has ever exacted upon our republic. And to refuse to hold our former president to account is, to my mind, to dismiss their responsibilities as representatives of the federal government.”

Bob Bauer, a professor at New York University School of Law who was White House counsel under Obama and a senior adviser on Biden’s campaign, said Republican senators are effectively seeking to establish a “loophole” in the critical constitutional mechanism for holding presidents accountable for high crimes and misdemeanors.

That might block conviction, Bauer wrote in a New York Times op-ed Tuesday, “but the moment should not pass without calling out in clear terms the damaging constitutional precedent that this outcome will produce.”