It was just two months ago that Sen. Bob Corker (R-Tenn.) made a bold declaration about the Republican tax plan on “Meet the Press.” The retiring Republican senator told NBC’s Chuck Todd, “If it looks like to me, Chuck, we’re adding one penny to the deficit, I am not going to be for it, okay? I’m sorry. It is the greatest threat to our nation.”

Despite heavy partisan pressure, the Tennessean stuck to his guns. When it came time for a floor vote on the Senate GOP’s proposal, literally every Republican fell in line and did what they were told do – except Corker. Faced with independent estimates that the Republican tax plan would add at least $1 trillion to the deficit over the next decade, Corker honored his pledge, kept his word, and opposed the plan.

With one more vote on the way – Congress still has to vote on the final tax package – the GOP senator suggested yesterday that his principles remain intact. “The deficit concerns certainly have not been addressed,” Corker explained. “My guess is it’ll be very difficult to resolve that component.”

Indeed, Republicans never even tried to “resolve” his concerns – but today he threw his principles out the window anyway.

Sen. Bob Corker, R-Tenn. – who had been the sole vote against the initial Senate bill, citing deficit concerns – also announced his support for the GOP plan on Friday.

Corker’s announcement followed word that Sen. Marco Rubio (R-Fla.), who announced yesterday that he wouldn’t vote for the plan unless his demands for an expanded child tax credit were met, also flipped from “no” to “yes” today after GOP leaders made some modest concessions in his direction.

And so, as things stand, how many of the 52 Republican senators are prepared to break ranks on a regressive and unpopular tax plan? Literally none.

I’ll confess, the Corker reversal surprises me a bit. His party didn’t really need his vote – as the recent vote on the Senate plan proved, the GOP package can pass without him – so he could honor his public commitment without imperiling his party’s agenda. I expected him to keep his word, if for no other reason, than to bolster his legacy.

But Corker decided to break his vow anyway. As the Tennessean enters the final year of his congressional career, this may be the last big vote he casts on a major piece of legislation, and he’s decided that vote will be an abandonment of his principles.

As for the nation’s interests, the odds of the tax plan passing are now nearing 100%. Recent lobbying has focused on finding senators who might be willing to join Corker in opposing the GOP bill, but with Corker giving up on his commitment, attention will now have to turn to individual U.S. House members.

The vote in the lower chamber will be held early next week.