At this point, Democrats seeking an impeachment process seem to be holding both leaders equally culpable for that process not moving forward.

In the wake of reporting that Trump encouraged the president of Ukraine to investigate former vice president Joe Biden — the leading contender to face Trump in next year’s presidential race — there has been renewed energy on the left to move forward with an impeachment vote. Many of the arguments to that effect are less focused on outcome than the need to undergo the process, a push for Congress to at least try to deploy its constitutional check on the executive branch.

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The politics of an actual vote, though, remain tricky.

First, there’s the often-repeated analogy of the investigation into Richard M. Nixon. As the Senate probed the details of the Watergate break-in and the Nixon White House’s effort to cover up the truth in that case, Nixon’s approval rating dropped quickly. The theory, then, is that the outcome of an impeachment vote for Trump isn’t a certainty, since he might similarly lose support as revelations emerged.

The problem with that idea is twofold, as we’ve written before. The first problem is that Nixon was president in an era in which presidential approval ratings fluctuated much more significantly. For the past decade or so, that hasn’t been the case; partisans support a president from their party and oppose a president from the opposition, while independents meander around the middle. Trump’s approval rating has been stuck in a narrow range from the outset of his presidency, a function of being consistently well-liked by Republicans and opposed by Democrats.

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That support from Republicans is linked to the second problem with the Nixon analogy. Trump has a robust universe of supporters who rush to his defense, from elected Republicans (worried about antagonizing his fervent support base) to members of the conservative media (eager to keep that support base tuning in). We’ve already seen how Trump’s supporters are spinning the Ukraine issue; if past is precedent, those defenses of Trump will help bolster his support from Republicans.

Setting aside the Nixon example, an impeachment push isn’t necessarily going to sail through the House in the first place.

The Washington Post has been tracking which members of the House Democratic caucus support impeaching Trump. Since the beginning of the year, the number who do has increased significantly, picking up speed in earnest after the release of former special counsel Robert S. Mueller III’s report on Russian interference in the 2016 election. (One Republican joined the impeachment calls before leaving his party in July: Justin Amash (I-Mich.).)

Notice that bar on the far right, though. It signifies the number of votes needed to have a majority in the House. And although more than half of Democrats support impeachment publicly, nearly all of the caucus would need to support such a vote if Republicans remain unified in opposition to it. The light blue on that chart indicates Democrats who haven’t weighed in on impeachment or who are opposed to it — but nearly all of those members explicitly oppose an impeachment effort.

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That’s a lot of ground to make up. And it’s a lot of ground to make up among a group of Democrats that is both more ideologically conservative than the caucus on the whole but also represents less-heavily Democratic districts.

Over time, the average partisanship of the districts of House Democrats who support impeachment has moved toward the middle, according to Cook Political Report’s assessments of the partisan voting behavior of every district. Similarly, the ideology of House Democrats who support impeachment, as calculated by VoteView, has trended toward the middle.

As less liberal Democrats from less solidly blue districts join the impeachment effort, the ideology and partisanship of those who still oppose impeachment have also moved away from the more liberal pole. If you have three Democrats from districts that are rated D+30, D+20 and D+10 by Cook, the D+20 Democrat joining the D+30 Democrat in supporting impeachment moves the average district rating for impeachment supporters from D+30 to D+25 and the average of opponents from D+15 to D+10. The same thing is happening across the caucus.

That isn’t to say that there aren’t Democrats from across the spectrum signing up for impeachment. As the year has progressed, the Democrats joining calls for impeachment have represented various parts of the caucus, as below. The graphs below show the ideology and district rating of each House Democrat. Districts get less safe for Democrats as we move from left to right; representatives get more liberal as we move from top to bottom.

As of writing, though, there are still 28 Democrats from Republican-leaning districts who oppose impeachment, compared with seven from districts where the Democrats have at least a 30-point advantage who do. Of the 97 Democrats who don’t support impeachment, 68 are in districts less friendly to Democrats than the caucus average, and 59 are themselves less liberal than the average House Democrat.

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In another sense, though, that suggests that there is still a decent amount of low-hanging fruit for Democrats.

Consider the most recent version of the above chart. The yellow dots are the Democrats who’ve embraced impeachment since July; notice that they’re fairly centrally scattered throughout the caucus.

But there are a few dark circles — Democrats who oppose impeachment — who are outliers.

Like Rep. Sylvia Garcia (D-Tex.) who told CQ Roll Call in May that “we should try all avenues and exhaust them to try to reach an accommodation” on getting information from Trump — short of impeachment. Her district has a 19-point advantage for her party, and she’s identified as one of the most-liberal-voting members of the caucus.

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Or Rep. Karen Bass (D-Calif.), who sits on the House Judiciary Committee. A spokesperson for Bass confirmed to The Post in July that she opposes impeachment, despite her coming from one of the safest Democratic districts in the country.

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If Democrats pick up all of the Democrats who are either more liberal than the caucus average or who represent safer districts than the caucus average (including both Bass and Garcia), that’s another 45 votes for impeachment.

Bringing the total to 184 including Amash — 34 votes short of a majority in the House.

It’s possible that the look-what-happened-to-Nixon argument will be more effective on House Democrats. After all, as more has been learned over the past several months, many Democrats have been newly convinced. But there are still 46 Democrats who represent districts that are rated D+5 or more Republican — all of whom, presumably, would like to be reelected next year. That’s the calculus Pelosi is making: Don’t make these representatives vote on a still-unpopular Trump impeachment.