(CNN) Just as President Donald Trump has tried to rewrite the rules of US politics, the move by Democrats to impeach him offers something completely unprecedented.

Only two presidents have ever been impeached: Andrew Johnson in 1869 and Bill Clinton in 1998. Richard Nixon resigned in 1974 when it was clear impeachment was coming.

In each of the previous impeachment efforts, senators crossed party lines. There's no indication of that happening right now.

Quite the opposite. In fact, Sen. Lindsey Graham, a South Carolina Republican who pushed forcefully to impeach Clinton in the 1990s and used to be an outspoken Trump critic, is now contorting himself to defend the President , rejecting the idea that there's anything at all wrong with Trump asking foreign leaders to help generate dirt on former Vice President Joe Biden.

One former Republican, Rep. Justin Amash of Michigan, who was basically driven from the GOP for breaking with Trump over special counsel Robert Mueller's Russia report last spring, has sounded open to the impeachment inquiry.

Some Republicans, notably Sen. Mitt Romney of Utah, have expressed some concern about Trump's call with the Ukrainian President. Sen. Charles Grassley of Iowa said the whistleblower deserves to be heard.

"No one should be making judgments or pronouncements without hearing from the whistleblower first and carefully following up on the facts," Grassley wrote in a statement Tuesday. "Uninformed speculation wielded by politicians or media commentators as a partisan weapon is counterproductive and doesn't serve the country."

This is not to say previous impeachment efforts weren't hyperpartisan. It was Ken Starr's shifting focus on a cascade of alleged scandals that led Hillary Clinton to allege a " vast right-wing conspiracy " against her husband -- though the charges for which Bill Clinton was ultimately impeached -- perjury and obstruction of justice -- were serious, especially in the hindsight of #MeToo and considering his power over White House intern Monica Lewinsky.

But they did not involve using the power of the federal government to undermine his opponents. And neither impeachment article that passed the House got even a simple majority in the Senate, much less the two-thirds needed to pass in that body.

That's because there were Republican senators who ultimately opposed Clinton's removal from office after the House impeached him. Conversely, there were Democrats in the House who supported his impeachment.

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In an interview Tuesday on CNN, Carl Bernstein, whose reporting helped uncover the Watergate scandal, said the allegations against Trump share some similarities with those against Nixon, who resigned just before he would have been impeached by the House of Representatives.

"Both presidents wanted to undermine the very basis of American democracy, which is free elections," Bernstein told Jake Tapper. "In Nixon's case, it was a campaign of political espionage and sabotage intended to get the Democrats to nominate George McGovern, his weakest possible opponent, as opposed to his strongest opponent, who would have been Sen. [Edmund] Muskie of Maine, who was crippled by this political espionage and sabotage."

But there is a glaring difference -- at least so far: Ultimately, it was Republicans turning on Nixon that made him resign.

Nixon's abandonment started with Saturday Night Massacre

The resolution to begin an impeachment inquiry against Nixon, passed after the infamous Saturday Night Massacre firing of Justice Department officials, passed 410-4, with overwhelming bipartisan support. Later, when the House Judiciary Committee took its vote against Nixon, seven Republicans on the committee voted for at least one of the three articles of impeachment against him.

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"Here's the big difference between Watergate and what we're seeing now," said Bernstein. "If Republicans are willing to go along with this, it is going to change our history. Because Republicans became the heroes in Watergate who finally said we cannot tolerate a corrupt President who undermines our electoral system."

Presidential historian Douglas Brinkley agrees, and pointed out that it was a group of Republican senators who told Nixon he was losing the party.

"During Watergate, eventually you got Republicans, whether it was Barry Goldwater or Howard Baker and others, standing up to the power of Nixon, saying he lied," said Brinkley on CNN.

A profile in courage saved Andrew Johnson

In the case of Johnson, it was Republicans standing up to the power of their fellow senators. Johnson was a Southerner who remained loyal to the Union during the Civil War and was chosen as running mate by Abraham Lincoln to appeal to Southern Democrats when he ran for reelection.

Johnson took over after Lincoln's assassination but frustrated the Senate and Congress with his efforts to stand in the way of "Radical Republicans" who favored a more punitive Reconstruction effort and more protections for freed slaves. Congress overrode his veto and passed the Tenure of Office Act to curb his ability to fire Cabinet officials. He ignored it and fired Secretary of War Edwin Stanton so Congress, which had mostly Republicans in the period before Southern states had earned back their congressional representation, impeached him.

Seven Republican senators voted against the rest of their party and against removing Johnson from office. Sen. Edmund Ross of Kansas is generally considered to have cast the deciding vote to acquit Johnson.

There are some allegations that Ross may have essentially been bribed or he didn't want to elevate the ranking senator at the time to the presidency. Ross got the hero treatment from John F. Kennedy in his book "Profiles in Courage." His vote helped preserve the modern, powerful presidency. The Supreme Court said the Tenure of Office Act was unconstitutional in 1926 . But Johnson's presidential career was over after he was impeached, as he lost the Democratic Party's nomination for the 1868 election. He ultimately won back his old Senate seat from Tennessee.

The Union survived both Civil War and Johnson's impeachment and emerged with a strong executive. Trump, drawing accusations of tone deafness and false equivalence, has tweeted a quote suggesting there could be a Civil War-like rift in in the country if he were impeached.

The low bar of the Clinton example

When Clinton was impeached, he enjoyed protection from some Republicans despite the defection of a number of Democrats. No Democrats supported removing him from office in the Senate, but several supported his impeachment in the House. Enough Republicans broke ranks in the House to defeat two of the four articles of impeachment OK'd by the House Judiciary Committee.

In the Senate, five Republicans broke with their party to oppose removing Clinton from office. Two, Jim Jeffords of Vermont and Arlen Specter of Pennsylvania, ultimately left the party. Only Sen. Susan Collins of Maine remains a Republican and is still in the Senate from that vote.

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As with Clinton, the effort to impeach Trump, while just getting started, seems at this point doomed to fail. While Senate Majority Leader Mitch McConnell has dispelled the idea that he could simply ignore impeachment and confirmed that the Senate would conduct some sort of trial if Trump is impeached, it's hard to imagine the necessary 20 Republicans turning on Trump. Impeaching a president in the House requires only a simple majority of 218, which Democrats have. Removing a president from office requires a two-thirds Senate majority, usually 67.

In that way, Trump's impeachment is more like Clinton's, in that it is unlikely to succeed in removing him from office.

"We've simply never had a case before where the removal of a president was so well justified -- while at the same time so obviously unlikely to happen," wrote the Rutgers historian David Greenberg in Politico recently.

CORRECTION: This story has been updated to correct the date of the election after President Andrew Johnson's election and Sen. Edmund Muskie's first name.