It’s becoming increasingly clear that the fate of the economy may turn on the answer to that one question.

As of last night, some 90 House Republicans had signed on to a letter being circulated by Tea Party Rep Joe Walsh that calls for the GOP leadership to oppose any vote on the McConnell escape hatch proposal to transfer control of the debt ceiling to the president.

So can the McConnell plan pass the House? Jed Lewison has the most extensive look at the math I’ve seen yet:

There are currently 432 members of the House (with three vacancies), so you need 217 votes to pass a bill. There are 239 Republicans, so if between 90 and 100 of them have ruled out supporting a “Plan B,” Boehner’s best case scenario is getting about 140 or 150 votes from his conference. That will leave him in the range of 70 to 80 votes shy of raising the debt limit.

Obviously, those 70 to 80 votes must come from Democrats. In April, 81 of them voted for the budget deal compromise with President Obama, but House Democrats are going to be less eager to vote to raise the debt limit than they were to pass a funding bill.

...if Boehner keeps on losing Republicans, there’s going to come a tipping point where loading up the debt limit vote with spending cuts [to win GOPers back] will make it increasingly difficult to win Democratic votes to pass it, because there’s only so many Blue Dogs out there who actually want to cut spending. At the same time, he’ll risk losing Republican and even some Blue Dog votes if he reduces the level of cuts. If and when we reach that point, the question will be whether he is able to deliver votes from members who may be unhappy with the level of spending cuts...If he can’t, the debt limit won’t be raised.

Given the mounting signatures on the Walsh letter, the momentum is moving in exactly the wrong way in the House. That, plus the lack of clarity about where House Dems stand on the McConnell plan, mean it’s becoming increasingly clear that there will have to be a real shift among House Republicans in order for the debt ceiling to be raised. The only way that’s going to happen is if there’s a major “WTF do we do now” moment among House Republicans when they suddenly find themselves peering into the abyss — when they suddenly realize they are about to take the blame for allowing default and cratering the economy.

How will they react when this moment arrives? As Ezra Klein notes, it’s still unknowable: “what no one quite knows is what the House GOP will accept when the clock is one minute from midnight, or, in more pessimistic tellings, the Dow is 1,000 points below whatever it was at the day before.”

This “WTF do we do now” moment can take one of two forms: Either House Republicans agree to new revenues, or they agree to the McConnell proposal. But one of those two things has to happen. And that moment is almost upon us.