WASHINGTON — Happy Impeachment Day.

Donald Trump is just hours away from becoming the third president in American history to be impeached by the House of Representatives, a humiliating designation that will permanently tarnish his legacy no matter what else he does.

In that sense, Trump’s about to outdo even disgraced former President Richard Nixon, who resigned in 1974 before the House had time to formally impeach him.

The crucial vote is now expected to take place Wednesday evening following hours of fiery rhetoric on Capitol Hill from both Trump’s critics and his defenders.

Trump erupted in rage at House Speaker Nancy Pelosi on the eve of his impeachment Tuesday, releasing a six-page letter that amounted to a barely-controlled presidential temper tantrum, in which Trump insisted the vote would sully Pelosi’s place in history, rather than his own.

“You have cheapened the importance of the very ugly word, Impeachment!” Trump wrote. “It is a terrible thing you are doing, but you will have to live with it, not I!”

Democrats’ impeachment case boils down to accusing Trump of attempting to cheat in the 2020 election by pressuring Ukraine to announce investigations of Democrats, including 2020 rival Joe Biden, and then blocking Congress from investigating what he did.

Those accusations have now crystallized into two distinct articles of impeachment: one for abusing his power and another for obstructing Congress.

With Trump’s legacy in the balance, here’s what you need to know.

What today will look like

House Democrats will kick off Wednesday morning by approving guidelines governing the procedure for the big vote, which were laid out by the Rules Committee Tuesday night.

The House will then embark on several hours of rhetoric, argument and bombast — none of which is likely to change the final vote or pretty much anything else, until they finally zero in on approving the two articles toward the end of the day.

Republicans may bog down the process by offering up amendments that are unlikely to be approved by the Democrat-controlled House. That could slow things down and postpone the vote until later in the evening.

After the articles of impeachment are approved, the House will need to pass yet another measure sending the whole thing to the Senate for a trial and also naming impeachment managers, who will act like prosecutors in the Senate proceeding.

Right now, Trump’s impeachment in the Democrat-controlled House appears a deadlock cinch — likewise his subsequent acquittal in the GOP-controlled Senate. That bureaucratic two-step will leave Trump in office, enraged but still president, and raise the curtain on his fight for reelection in 2020.

But it doesn’t mean the impeachment battle lacks drama. Case in point: The quandary faced by House Democrats from areas where Trump remains popular.

House Democrats united

There are 31 House Democrats in districts Trump carried in the last presidential election, including 21 freshmen. Basically every one of them is expected to vote to impeach him.

That’s a remarkable show of unity — and either political bravery or kamikaze foolhardiness, depending on who you ask. These Democrats are clear-eyed about the risk of backing impeachment. But they’re doing it anyway.

Of the 31 House Democrats in Trump districts, 28 have said they’ll vote for impeachment, and most expect Rep. Ron Kind (D-Wis.) to join them.

That leaves just two (out of 233 House Democrats) who are now expected to vote against impeachment. And one isn’t even really a Democrat anymore. Rep. Jeff Van Drew (N.J.) is expected to leave the Democratic Party, and most of his staff have already resigned in protest. Veteran Rep. Collin Peterson (D-Minn.), whose district Trump won by 30 points, is the only Democrat who’s sent a clear signal he’ll vote against impeachment, while Rep. Tulsi Gabbard (D-Hawaii) is also making noises about opposing it.

House Republicans are expected to vote en masse against impeachment. That means there’s a real possibility that Peterson is the only lawmaker to break with his party on the final impeachment vote.

...And here comes the Senate trial

Senate Majority Leader Mitch McConnell (R-Ky.) basically declared he wouldn’t seek a fair Senate trial on Tuesday. The real question is how his more establishment-minded senators decide to go along with him.

“I’m not an impartial juror. This is a political process,” McConnell said during his weekly press conference Tuesday after rejecting Democrats’ request to call Trump administration officials as witnesses during the Senate trial.

McConnell plans to knock out a quick Senate trial in January, and dismissed calls to design a bipartisan process that everyone could live with, like what happened during Bill Clinton’s impeachment trial two decades ago.

If Democrats hope to change the rules at all and get what they see as a fair shake, they’ll need some help. That means wooing four GOP senators on any vote — with the most likely targets being Sens. Susan Collins (R-Maine), Mitt Romney (R-Utah), Lamar Alexander (R-Tenn.) and Lisa Murkowski (R-Alaska). Collins has already criticized Democrats’ approach, though, and that’s a high bar for them to clear.

It remains to be seen how the Senate sets up its impeachment trial — but for now, it looks like the majority Republicans will have the upper hand.