For the first time in a generation, lawmakers could not agree on a federal spending bill before the fiscal clock wound down, forcing the government to shut down.

Congress had until midnight Monday to approve a continuing resolution to keep the government open. But Senate Democrats balked at House Republicans' repeated insistence, laid out in a handful of bills, to delay or snip at ObamaCare in exchange for funding the government. And House Republicans wouldn't accept bills stripped of that language sent to them by Senate Democrats.

So what happens now?

Senate Democrats have insisted they will not pass anything that undercuts ObamaCare, and even if they did, President Obama would quickly veto it. "You don't get to extract a ransom for doing your job, for doing what you're supposed to be doing anyway, or just because there's a law there you don't like," Obama said Monday, hours before the shutdown.

It's a good bet that Obama will stick to that strategy, and not only because conceding to Republicans would establish a pattern in which a single branch in Congress could make demands of the executive branch by holding the economy hostage. As the shutdown began, Obama enjoyed the benefit of public opinion.

A Washington Post/ABC poll released on Monday showed that only one in four Americans approved of how Republicans approached budget negotiations. If the backlash against a shutdown picks up, Republicans could feel even more heat.

Speaker of the House John Boehner (R-Ohio), under pressure from a hard core of Tea Party conservatives, has steadfastly pushed for concessions from Democrats. Yet as the fight dragged on, Boehner has increasingly lost control of his caucus. While conservatives have pushed Boehner around, moderate Republicans are now accusing him of going too far to the right.

Rep. Devin Nunes (R-Calif.) called his colleagues pushing for the shutdown "lemmings with suicide vests."

"They have to be more than just a lemming," he said. "Because jumping to your death is not enough."

Facing opposition from both sides, Boehner may now have no way out but to make a deal with Democrats, as he did in the sequester fight.

Here are MSNBC's Benjy Sarlin and Suzy Khimm on that point:

Moderates and conservatives alike are starting to rebel, leaving Boehner exposed on the left and the right on a vote that’s supposed to show his strength in the final hours before a shutdown. That means Boehner will head into battle against a united Democratic front hobbled by an ugly civil war raging in his party. [MSNBC]

There are some 175 Republicans now ready to vote for a clean budget, according to the Washington Examiner's Byron York. Boehner has so far been boxed in by tricky legislative math and his need to appease the holdouts — York explains in great detail how 30 or so entrenched conservatives have been able to hijack the entire debate within the party. But Boehner could pass a clean budget with ample GOP support if he finally brought one to a vote.

Adding urgency to the debate, the federal government will hit the debt ceiling in just two weeks. Republicans have threatened to take the economy hostage in that fight, too, in order to, once again, try to dismantle ObamaCare.

Boehner could bring a clean budget to a vote to end the shutdown, while simultaneously trying to allay conservative anger by promising to shift the fight to the debt ceiling. However, a plan laying the groundwork for that very idea fell apart last week when Republicans couldn't muster enough support among themselves to approve a list of demands.

Already on Monday, stocks fell in anticipation of the shutdown. If the shutdown drags on — and especially if the fiasco threatens to send the government crashing through the debt ceiling — they will almost certainly slide further. That could add extra incentive for Republicans, once known as the party of big business, to back down and save the ObamaCare fight for another day.

The last government shutdown in 1995 and 1996 lasted a total 28 days. How long this one will last is anyone's guess at this point.

Sizing up the long odds facing his party, Republican Sen. Tom Coburn of Oklahoma — whom National Journal ranked as the most conservative member of the Senate — predicted last week that Republicans would cave within a week once a shutdown hit. The GOP, he predicted, would "fold like hotcakes."