I know you guys are bored to tears with fiscal cliff stuff by now, but after weeks of wandering in the news desert, we’ll finally have some suspense tonight. Cantor was talking tough this morning about how they supposedly have the votes. According to The Hill’s whip count, though, there are already 23 Republican defections — and Boehner can only afford 24. They may be lowballing it, too. They’ve got two reps from South Carolina listed as no’s, but there may be more to come:

Mick Mulvaney is a no on Boehner bill and suggests entire SC delegation is as well #demint

— Russell Berman (@russellberman) December 20, 2012

SC has seven House seats so if Berman’s right then Boehner’s already short on votes with more defections possible before tonight’s vote. (Erick Erickson’s own whip count, taken this morning, pegged the number of GOP no votes at 34.) Some of his opponents are so entrenched, in fact, that they’ve taken to going on MSNBC in order to … convince hardcore liberals that we need to dramatically slash spending, I guess? Anyway. What’s a Speaker whose credibility is on the line to do? Just what you’d expect:

Good R source: "Right now there are btween 30-40 R no votes. But I'm told ppl being threatened with everything under the sun…" [1/2]

— Joshua Green (@JoshuaGreen) December 20, 2012

"– gavels, chairs on committees etc. By the time the arm twisting is done tonight, I think Plan B passes barely." [2/2]

— Joshua Green (@JoshuaGreen) December 20, 2012

They’re voting on this thing tonight instead of in the afternoon to give Boehner, Cantor, and McCarthy time to twist arms and make threats. What happens if it doesn’t work, though? Will they still hold the vote and risk humiliation, on the theory that even a no vote might put some pressure on Obama to up his offer on spending cuts in order to push more conservatives into the yes column on a grander compromise with the White House? Or will they yank the bill, which would arguably be even more embarrassing?

Ramesh Ponnuru lays out the argument for voting yes, even though he’s not a fan of Plan B in all its particulars:

If it works out, it will go something like this: House Republicans pass Plan B, showing that they are not protecting millionaires, are taking steps to protect the middle class, and have moved on from the extend-all-the-rates position they took during the summer. The onus will then (again, if things work out) be on Senate Democrats to budge too, perhaps by passing a bill that extends tax cuts for everyone making less than $500,000 and limits tax increases on dividends. At that point we would see what Reid could pass, there being some dissension in his own caucus.

Then the stage would be set for a final bill that extends all the tax cuts for everyone making less than, say, $750,000 a year. Some people are asking how a deal can pass the House with a threshold below $1 million if Boehner is having so much trouble getting Republican votes for a bill with that threshold. The hope has to be that a lot of House Democrats—who have not yet had a chance to vote for a bill that extends the middle-class tax cuts—would support a deal, and therefore Boehner wouldn’t have to round up as many conservative votes for it.

Right. If Plan B fails, there are two options left: Some sort of deal with Obama in which Democratic votes replace Republican defectors, or a Republican cave on tax rates under tremendous pressure after we go over the cliff. (If you think an Obama cave is more likely, explain to me why.) But if the bill does fail, why would Obama offer Boehner new concessions and rescue him from a major political embarrassment? A talking point about “obstructionist Republicans” obstructing their own Speaker will have fallen into his lap — and he’s going to give it away by upping his offer in order to strike a deal before New Year’s? Hmmmm.

T-minus three hours and counting. Here’s Boehner at today’s presser summing up the messaging value of Plan B in two blunt sentences. “I did my part. They’ve done nothing.”