Contrary to what you might have heard. Republicans largely stuck with Richard Nixon through most of the Watergate scandal despite the threat of impeachment. Photo: Wally McNamee/Corbis via Getty Images

As impeachment fever again seizes certain elements of the Democratic Party, it’s a good time to remind ourselves how vanishingly scarce the odds are that such an effort will result in the removal of Donald Trump from office. The reason is extremely simple: Blocking a conviction for impeachable “high crimes and misdemeanors” requires a mere 34 senators. Between now and the 2020 elections 53 senators (barring resignations) will belong to the party of a president whose rank-and-file voters adore him despite massive evidence of his crudeness, corruption and moblike habits and associations. One can imagine vaguely that Robert Mueller or a congressional investigation will turn up malfeasance so gross that Senate Republicans will defect en masse. But there’s little or no historical basis for believing that’s going to happen in any realistic scenario.

There have been two successful presidential impeachments in U.S. history (neither of which led to a Senate conviction), and one near-impeachment that produced a presidential resignation. In all three cases, Congress was controlled by the president’s opponents. That’s how things got as far as they did.

Indeed, the main impetus for Andrew Johnson’s impeachment in 1868 was his alleged betrayal of, and unambiguous split from, the Republican Party which had (under the guise of a wartime Union ticket) lifted him to the vice presidency. In 1966 Johnson campaigned (mostly unsuccessfully) for Democratic congressional candidates, even as he battled Congress through a long series of vetoes and refusals to implement what he considered to be unconstitutional laws. And so by the time Republicans moved to impeach him, he had functionally become a Democrat (as he had been his entire career up until 1864). The impeachment vote was on strict party lines, and every Senate Democrat (along with 10 Republicans) voted to acquit him. The atmosphere of polarization, which surely exceeded even today’s, drove Democrats to stick with Johnson even though it did not have the power to save him.

So, too, did Richard Nixon’s Republicans, until very late in the Watergate crisis when it was obvious he had cooked his own goose. And it’s noteworthy that the Republicans who defended him the longest were the very conservatives who had up until then so often deplored his ideological heresies such as wage and price controls, recognition of China and détente with the Soviet Union. As Jeet Heer observes:

Prior to Watergate, Reagan had occasionally been critical of Nixon as being too liberal. But once the scandal broke, Reagan became Nixon’s most prominent supporter. This helped Reagan win the loyalty of many Republicans, who shared his dismay that Nixon was being maligned by the press and by politicians from both parties.

In a look back at scandals affecting Nixon, Reagan and Bill Clinton, FiveThirtyEight concluded that core ideologues tend to become more, not less, loyal during crises:

We went back and looked at key congressional votes during three relatively recent periods in which a president was accused of wrongdoing: Watergate (Richard Nixon), Iran-contra (Ronald Reagan) and the Monica Lewinsky scandal (Bill Clinton). Two trends stick out. First, partisanship still matters. And in a big way. Second, when defections do come, they’re more likely to come from the centrist wing of a party.

While we tend to look back at Watergate and perceive a gradual national consensus over Nixon’s criminality, it came slowly to conservative Republicans, right up to and sometimes beyond the release of the “smoking gun” tape that proved Nixon’s complicity in the Watergate cover-up, which he had repeatedly denied. In the end only a minority of Republicans (mostly moderate-to-liberal Republicans) on the House Judiciary Committee voted for articles of impeachment, and soon the GOP was embroiled in a presidential nominating contest between the man (Reagan) who defended Nixon to the last ditch and the man (Ford) who pardoned him.

The Republican impeachment of Bill Clinton was also the product of polarization, and despite widespread criticism of the president by fellow-Democrats for his behavior in the Lewinsky scandal, very few of them were willing to do more than censure him. Only five House Democrats (all outspoken conservatives) voted for articles of impeachment, and no Senate Democrats voted to convict him.

So what lessons should we take from history about prospects to impeach and remove Trump? There are three: 1. barring some truly amazing revelation (maybe videotape of Donald Trump taking money from a Putin agent) a Republican-controlled Senate is not going to remove him from office; 2. conservative domination of the president’s party will increase its solidarity with him; and 3. the atmosphere of partisan and ideological polarization will make defections from Trump even less likely.

Having said all that, there’s a case to be made that House Democrats should impeach Trump even if they know for a fact it won’t result in a conviction. That’s basically the calculation Republicans made in impeaching Clinton in December 1998. Indeed, they made that decision in the teeth of massive evidence that voters had just rejected impeachment in the November 1998 midterms.

Democrats might proceed with impeachment to redeem campaign promises, to keep the party base “energized,” or simply because they believe it’s the right thing to do, and serves as a warning to future corrupt and conniving pols who aim high. But they should be aware that it is likely to increase rather than diminish Republican solidarity with Trump at both the elite and grassroots levels, and will end with Trump still in office (all the more because leaving it could expose him to criminal prosecution). And if it matters to them, there’s absolutely nothing more polarizing than an effort to remove an elected president from office.

By the time Andrew Johnson was acquitted in 1868, he was already on his way to a failed bid for the Democratic presidential nomination (itself an empty prize that year as Ulysses S. Grant swept to victory). Nixon and Clinton were in their second terms when efforts to remove them from office were undertaken. It’s very unlikely that an impeachment drive against Trump will keep him from securing his party’s presidential nomination a second time. How it would affect his standing in the 2020 general election should probably be a central question for Democrats as they ponder that dramatic step.