Trump's nightmare scenario of being impeached, convicted, then barred from public office for life is looking more and more likely

• Shorn of GOP support after the riot at the US Capitol, President Donald Trump is facing a three-part reckoning in Congress.
• The House is due to vote Wednesday on impeaching him for his role encouraging last week's violence.
• Republicans have been moving in support of such a move, making it likely to succeed.
• In the Senate, which would conduct Trump's trial, once-loyal Republicans are also said to be wavering, raising the prospect that he could also be convicted.
• An even harsher reckoning could come after a conviction, if Congress voted to bar him from office, ending any hope of a 2024 run.
• It remains to be seen whether the GOP revolt against Trump will be widespread enough for this to happen — but the prospect is looming larger than ever before.

With Senate Majority Leader Mitch McConnell and Rep. Liz Cheney moving in support of President Donald Trump's impeachment, a nightmare scenario for the president is moving closer to reality.

The congressional GOP was vocal in its support of Trump the first time he was impeached, with only one Republican senator, Mitt Romney of Utah, voting to convict him on one of the two charges over his dealings with Ukraine.

This time the situation is very different.

With Trump set to become the first US president to be impeached twice, Republican support for him is collapsing. This opens him up to the possibility of three reckonings, each more damaging than the last:

• Trump could be impeached in a House vote Wednesday - which appears almost certain to happen.
• He could be convicted by the Senate, a scenario that had been viewed as highly unlikely but is now being discussed as a real possibility.
• If Trump is convicted, Congress can move to bar him from public office for life.

Conviction in a Senate impeachment trial requires a two-thirds majority, meaning Democrats would need at least 17 Republicans to vote with them for Trump's removal.

According to The New York Times, as many as 12 have seemed open to voting to convict. That count came before McConnell and Cheney made their positions known, which could prompt more Republicans to follow.

A Wednesday-morning article from Axios, citing Republican sources, also spoke of a wholesale move in the party against the president.

An impeachment conviction would not automatically bar Trump from holding office, but a further move could.

The Senate could follow a conviction with another vote to block him from doing so, requiring a simple majority to pass. It would end any hopes of Trump seizing back the presidency with a run in 2024.

There are complications, though.

President-elect Joe Biden is set to be inaugurated on January 20. For an impeachment conviction to remove Trump from office, the Senate would have to finish its work within a week.

Traditionally, impeachment trials are drawn-out affairs, with scores of witnesses called and extensive deliberation.

McConnell also indicated last week that a trial could not be held before January 20, as Congress will be in recess.

Overturning this would require a unanimous Senate vote, an unlikely prospect given the pockets of Senate support for Trump, as noted by the legal expert Markus Wagner.

But a trial could still take place later, convicting Trump even after he has left office.

Such a trial could take place with the Democrats in control of the chamber, thanks to the party's recent victories in Georgia's runoffs.

Sens.-elect Jon Ossoff and Raphael Warnock are due to take office by late January, which would tilt the odds toward Democrats and give them more procedural control.

Read more: GOP kicks Trump to curb after deadly Capitol insurrection, leaving president to fend for himself during his historic second impeachment

Though some experts believe that such a trial would be unconstitutional, others say there is nothing in the Constitution expressly forbidding it.

"Once an impeachment begins in the House, it may continue to a Senate trial. I don't see any constitutional problem with the Senate acting fast or slowly," Michael Gerhardt, a law professor at the University North Carolina at Chapel Hill, told NBC News.

Another obstacle might be the widespread support Trump still has among grassroots Republicans.

Having been ejected from Twitter, Trump will no longer be able to bully and cajole senators on social media. But the lingering prospect of his supporters turning against them could be enough to discourage them from voting to convict the president.