The latest estimate from the majority whip’s office is that the House will vote by 8:30 tonight on the crucial piece of John Boehner’s Plan B — a bill that would extend the Bush tax cuts for income up to $1 million.

Full background on that bill here, and the analysis in that piece pretty much still stands.

Except now a huge deal of uncertainty hangs over that vote.The idea of Plan B is and has always been that if Republicans can pass a bill making the Bush tax cuts permanent up to everyone’s first million dollars, they can foist blame for going over the fiscal cliff on to Democrats, and thus, perhaps, get President Obama to offer up more concessions on a tax and spending deal.

But the strategy was too cute by half. Obama’s not caving. He’s threatened a veto. Democrats say the bill won’t even get a vote in the Senate. And so anti-tax Republicans are being asked to vote for a tax increase that will never become law, to provide Boehner with leverage in negotiations over a budget deal those same Republicans won’t ever vote for anyhow.

So Boehner has a whip problem. Thursday has been beset by delays, conservative grousing, and various other signs that internal headwinds might blow Plan B off course. Late last night, he had to promise conservatives a vote on a separate piece of legislation — to rollback the sequester and replace it with cuts to social programs and Wall Street reform rollbacks — as an enticement to vote for the millionaire tax.

That bill sneaked by on a 215-209 vote a short time ago. But the only vote that matters is the tax vote. Whether it passes or fails, the next step in this saga is for Obama and Boehner to determine whether there’s sufficient political will and enough time before the new year to continue negotiations on a larger measure. If it passes Boehner will use its existence to demand further spending and tax concessions from Obama.

But if it fails, or he has to pull the bill, he’ll have a fine mess on his hands. His three options will be an agreement with Obama that includes a much bigger tax increase, a Senate bill that raises taxes on income above $250,000, or the cliff. All bad from a GOP perspective. And he won’t even have the pretense that Democrats are to blame for the ensuing uncertainty.

Update: The House was scheduled to begin debating the millionaire tax bill moments after we published this post. Instead, Republicans placed the House in an unexpected recess, suggesting they lack the votes to pass Boehner’s bill. At least for now.