A partial government shutdown starting tonight is now looking like a certainty.

Twelve hours before spending authority lapses for all but essential federal operations, there is no sign that either side is willing to contemplate any concessions.

The only viable path to avert the shutdown is under the control of Speaker John A. Boehner, although this morning he gave no hint he was interested in pursuing it. “The House has done its work,” the Ohio Republican declared on the floor in reference to the continuing resolution that his unified Republican troops pushed through early Sunday. It would maintain sequester-level funding of all federal operations for a dozen weeks, delay the health law for a year and repeal a tax on medical devices.

The Senate will convene at 2 p.m., and all indications are that by this evening, the Democrats will be able to use their majority muscle to delete the Obamacare and medical device riders before sending the bill back once more to the House. There are no indications that Texan Ted Cruz or other conservative Republicans will try to prevent that from happening, knowing that delaying beyond midnight would shift responsibility for a shutdown onto them.

Even before the paperwork arrives from the Senate, though, Boehner would be able to get almost all the Democrats and several dozen Republicans to vote for a straightforward extension of existing spending levels — so long as it lasted only until the middle of October, which would align the appropriations battle with the deadline for raising the debt ceiling. The Senate can be counted on to quickly clear such a clean, short-term CR, which would then set the table for a climactic round of negotiations in search of deal that would bring both halves of the budget battle to a coordinated end.

For that scenario to play out, though, Boehner would need to conclude, at the last possible hour, that he can survive the outrage that would spill forth from the hard-line conservatives who dominate his caucus. He would have to persuade them that shouldering most of the blame for a shutdown in the immediate aftermath would weaken their long-term ability to gain any budget concessions or restrictions on implementation of the health law.

A more likely alternative is that, even if Boehner decides to risk assembling a bipartisan majority for a clean short-term CR, he will wait until the shutdown has been in effect for a few days. He would be calculating that the impact on federal services, the paychecks of 800,000 government workers and investor confidence is worth the potential payoff to him: Persuading the conservative hard-liners that the political stovetop they’ve just insisted on touching really was as hot as the leadership predicted it was, and that the fight they are spoiling for is better waged in conjunction with the debt debate.

The degree to which the tea party caucus seems determined to make tonight their symbolic last stand is undeniably high, however. The climatic House GOP caucus on Saturday, when the group last decided to stick with anti-Obamacare provisions knowing they would not survive the Senate, was punctuated with a cry of “Let’s roll!” from John Culberson of Texas — who later said he was alluding to the cry of United 93 passenger Todd Beamer.

The allusion was apt: The passengers who took over that plane on Sept. 11 expected to die, but concluded their heroic self-sacrifice was worth it to save big-time harm to the country.