Rep. Jeff Van Drew is reportedly leaving the Democratic Party on the week it votes to impeach President Trump. That solidifies something that has been evident this entire process: Impeaching Trump is a litmus test that defines which party you belong to.

That may sound obvious in the moment, but this impeachment inquiry, only the fourth of a president in the country’s history, will be particularly remembered for its partisanship, especially among Republicans.

A majority of Republicans in Congress have refused to say Trump has done anything wrong in the face of the evidence and testimony we’ve seen on the Ukraine matter. Many of them have bent facts to defend him or decried the way that Democrats are handling the impeachment inquiry process. The resulting message is clear: If you want to be a Republican, you need to support Trump unequivocally or close to it.

Extreme partisanship is the norm right now, even more than during the last presidential impeachment, in 1998.

During President Bill Clinton’s impeachment more than 20 years ago, dozens of Republicans voted against their leadership not to impeach him. Five Democratic members of Congress voted to impeach Clinton. Of those five, three eventually became Republicans. But that happened over time, not immediately after they stepped out of line with their party on this.

Today, we have two examples of the partisanship that impeachment of Trump has wrought. Several months ago, then-Republican Rep. Justin Amash left the party because he supported Trump’s impeachment, even though he’s one of the most conservative members of Congress. This was before the Ukraine matter was public — his rationale on impeachment was based on the findings in the Mueller report. His party leaders basically pushed him out the door.

Now Democrats are losing one of their own.

The Post’s Mike DeBonis reported this weekend that Van Drew has been considering and now is likely to switch parties. It came after Trump appeared to woo him in a long meeting Friday. Van Drew typically votes with Democrats on other big bills, but he doesn’t support impeachment.

We weren’t expecting Van Drew to vote for impeaching Trump, no matter which party he was in. He was one of two Democrats who voted against authorizing the impeachment inquiry in late October, which was seen as a potential preview for the actual impeachment vote happening this week.

And Van Drew likely has other motivations for switching parties; in his Republican-leaning district, which he flipped in 2018 for Democrats, he could have a hard time winning again as a Democrat, as impeachment has hardened each side and made crossover votes less likely. “He clearly feels that he needs to do this for political survival,” said GOP strategist Doug Heye in an email to The Fix.

In the process, at least six members of Van Drew’s staff resigned rather than work for a Republican. House Democrats’ campaign arm made a big deal about hiring those workers temporarily so they would still have jobs over the holidays. Here’s Democratic Congressional Committee Chairwoman Rep. Cheri Bustos (D-Ill.):

But whatever the reason Van Drew is leaving, getting him to switch to the Republican Party over this vote is a big get for Trump.

It won’t change the outcome when the House votes to impeach him this week, but it could help Trump undermine Democrats’ impeachment process by claiming it’s entirely driven by partisan Democrats and, therefore, shouldn’t be taken seriously.

“Partisanship defines almost every issue. National politics is becoming a team sport — what team you’re on defines which way you’re going on almost any given issue. A politician going against their party is increasingly treated like an athlete who scores on their own team. It’s in part why there’s so little bipartisanship, especially in national politics,” said Alex Conant, a GOP strategist, in an email.

Because Amash became an independent and didn’t join the Democratic Party over impeachment, Trump can technically argue there is bipartisan support for not impeaching him in the House, whereas it’s more likely that only Democrats vote to impeach him.

That display of partisan unity is important to Trump as his impeachment heads to the Senate for a trial next month.

It’s a show of strength to any Republican senators who might be thinking about voting to convict him on one or both counts, of abuse of power and obstruction of Congress. House Republicans didn’t budge, and they even got a Democrat to switch parties and join them. There might be as many as half a dozen Democrats who vote against one or both of the impeachment articles, worried about how it will play in the districts back home that Trump won in 2016. That’s another reflection of how loyal the Republican Party has become to Trump.

This is as partisan an impeachment process as there has ever been, and Van Drew’s last-minute party switch underscores it.