In the tale of 5 presidential impeachments, how will the latest one stack up?

When Republicans took power in the House of Representatives, they began trying to find evidence to make the case that President Biden had profited from the business dealings of his son, Hunter Biden.

They have not found that evidence, and struggled to bring public focus to their investigation — that is, until this week, when it was given the imprimatur of impeachment.

But this impeachment case is different — both in substance and in process — than the ones that loom large over the legacies of former presidents like Richard Nixon, Bill Clinton and Donald Trump.

"This current attempt to conduct an impeachment inquiry is unlike any others I think we've had in American history, because in the past there's always been some credible evidence of wrongdoing by the president that is part of the complaint against the president," said Michael Gerhardt, a professor of constitutional law at the University of North Carolina School of Law in Chapel Hill, who has testified as an expert witness in past impeachments.

"But in this situation, we don't have any credible evidence. And instead, this process seems to be what is sometimes called a fishing expedition," Gerhardt said.

House Speaker Kevin McCarthy has defended launching the impeachment inquiry in the absence of hard evidence, saying that finding evidence is the point of the inquiry.

The substance of the case against Biden is unclear

Under the Constitution, Congress can remove a president from office if it finds he or she committed "treason, bribery, or high crimes and misdemeanors." These are the inquiries from modern presidencies:

• Nixon was accused of obstructing justice by impeding the investigation into the Watergate break-in, abusing his presidential powers to harass his political enemies and refusing to cooperate with the House investigation.
• Clinton lied under oath about an inappropriate sexual relationship with a White House intern and was also accused of obstruction of justice for trying to keep it a secret.
• Trump, in his first impeachment, was accused of soliciting the interference of Ukraine in the 2020 presidential election and obstructing Congress.
• Trump, in his second impeachment, was accused of inciting an insurrection by lying about the results of the 2020 election that he lost and encouraging his supporters to head to the Capitol.

Past impeachments concerned things that happened during a presidency — not before

In Biden's case, House Republicans allege that Biden somehow benefited from the business dealings his son had in Ukraine and China, when he was vice president. The president and White House have denied this, and investigators have not turned up any smoking guns from witnesses or financial records.

That the alleged activity happened before Biden was even president makes this presidential impeachment unlike any other in U.S. history, said scholars who spoke with NPR. Keith Whittington, an impeachment expert at Princeton University, said impeachment is supposed to be reserved for grave offenses that present an ongoing threat to the country, making removal from office the only remedy.

"I think it's a much harder challenge to convince members of Congress that it's appropriate and useful to impeach an officer based on prior conduct that occurred prior to their holding office," Whittington said.

Past impeachments happened closer to the activity in question

In the case of Nixon, it took nearly two years from the time of the Watergate break-in to the formal launch of an impeachment inquiry. In the meantime, there were Senate hearings and a special prosecutor digging for evidence. By the time the House voted to launch the inquiry, the vote was widely bipartisan.

With Clinton, the House didn't officially launch its inquiry until independent counsel Kenneth Starr provided documentation the president had lied under oath and had taken steps to conceal the relationship, some 10 months earlier.

In Trump's first impeachment, House Speaker Nancy Pelosi announced there would be an inquiry days after she learned that the president had phoned Ukrainian President Volodymyr Zelenskyy two months earlier to try to strong-arm him to announce an investigation into his political rival, Joe Biden. His second impeachment happened a week after Trump supporters stormed the Capitol on Jan. 6, 2021 in an effort to overturn the results of the election.

The Biden inquiry breaks precedent in a number of ways — including that there is no known date when the offending activity occurred, because House investigators haven't produced evidence of such an event. But they are investigating the period when Biden's son Hunter was serving on the board of a Ukrainian gas company, almost 10 years ago.

"The distance now between where Joe Biden is today from the time period in which he's supposedly engaged in misconduct, is pretty substantial," said Gerhardt. "It's unique among presidents."

Some past impeachment inquiries began with a formal vote

There aren't many formal rules about the impeachment process itself — including whether it's necessary to first vote on whether to begin an inquiry. "That's mostly a question of politics more than it's a question of constitutional requirements," said Whittington.

In Biden's case, McCarthy launched the inquiry without having a formal vote — a step that had happened for Nixon and Clinton.

At the moment, McCarthy doesn't have enough votes within his own party to guarantee passage. Several Republicans from districts President Biden won in 2020 have said there isn't enough hard evidence to proceed with an inquiry.

McCarthy has pointed to the first impeachment of Trump as setting a new precedent for the process. Pelosi launched the process on Sept. 24, 2019. At the time, there were some reluctant Democrats who didn't want to be forced to take a tough vote.

Trump's White House counsel and allies including McCarthy objected vociferously, saying the inquiry wasn't legitimate because there hadn't been a vote. Ultimately, Pelosi held a vote on Oct. 31. All but two Democrats voted in favor of proceeding, and every Republican voted against it.

For Trump's second impeachment, there was no impeachment inquiry at all. The House moved straight to voting on the articles of impeachment a week after the Jan. 6 insurrection, in a push to impeach Trump before he left office on Jan. 20.

That impeachment has since been overshadowed by a House select committee that investigated the attack on the Capitol — recommending the Justice Department pursue criminal charges against Trump — and the indictment of the former president on four counts related to efforts to overturn the election results.

It's unclear how long these impeachment proceedings will take

Trump's first impeachment inquiry began a little more than a year before Election Day. House Democrats rushed it through in part to avoid a potentially politically unpopular event hanging over their own reelection campaigns.

It's not clear how Republicans plan to proceed for Biden, but Rep. Marjorie Taylor Greene, R-Ga. — an ally of McCarthy — has said it could be drawn out.

"What I actually want to see is, I want to see a very deep dive, a detailed investigation, no matter how long it takes, and it may take months and months," she told reporters. "It may go all the way to the November election."

An impeachment is a political process, not a criminal one. Getting an impeachment through the House requires a simple majority vote, which means the threshold for impeachment is what the party in power can bear politically, rather than any sort of legal standard.

"Technically speaking, if a party controls a majority of seats in the House, it has the power — the raw power — to bring about an impeachment," Gerhardt said.

Nixon resigned from office before the House could vote on impeachment. But in the cases of Clinton and Trump, once an impeachment inquiry was launched, it ultimately led to the House voting to pass the articles of impeachment.

No president has been removed from office by the Senate.

NPR's Daniel Wood contributed to this report.

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