Democrats are increasingly pessimistic about forcing real concessions from Republicans in the near term — for example, by using the threat of a government shutdown to force them back to the table — and are instead settling in for what will amount to a months-long war of attrition in hopes of ultimately getting Republicans to cave on new revenues.

Dems had initially hoped the sequester would force concessions, and when that failed, had begun to eye the threat of a shutdown as the next major point around which to try to force Republicans to deal. But signs are that Dems will end up agreeing with some version of the GOP approach of funding the government at sequester levels through September. It’s true, as Brian Beutler has reported, that House Dems will withhold support for the GOP bill to force Republicans to pass it on their own and that Senate Dems are pressing for some changes to mitigate the sequester’s impact on non defense spending. These things could lead to a shutdown. But Dems are looking beyond the shutdown deadline and are now shaping their strategy around the belief that they’re in for a much longer, drawn out war than they’d previously expected.

According to a senior Senate Dem leadership aide, Dem leaders have concluded that the GOP leadership is genuinely, irrevocably “entrenched” in their no-revenues posture. They believe the only hope for a long term compromise involving new revenues and entitlement cuts is to continue to try and pry loose Republicans who recognize that being the party of only spending cuts, and the economic damage they may cause over time, is a long term loser for the GOP.

“We do think the Republican leadership is completely entrenched,” this aide tells me. “If Boehner makes one false move, he loses his speakership. McConnell is paranoid about a primary challenge from the right.” However, this aide adds, other Senate Republicans “don’t want to be portrayed as the party of austerity. They know this isn’t a winning message long term.”

As Jonathan Chait details, there’s also the question of how long the GOP’s defense hawks will put up with sequester cuts, which are now likely to remain in effect for months. These lawmakers are already restive and signaling an appetite for compromise. Democrats will continue reaching out to these lawmakers — and others whose communities are being harmed by the sequester — in hopes of fomenting more dissent among them about the GOP leadership’s no-revenues-at-any-costs stance — and will continue telling them that a compromise on revenues is a reasonable way out. The most visible manifestation of this is the news that President Obama is privately talking to non-leadership Senate Republicans, but this approach goes well beyond the White House; it includes the Dem Congressional leadership, and will continue for weeks or months.

Dems are hoping the endgame in the fiscal cliff fight will prove as a model. Republican leaders drew a hard line but ultimately gave in after GOP lawmakers came forward and said the GOP stance was untenable, leading to a compromise in the Senate and, after a lot of drama, a vote for that compromise in the House that passed with a lot of Dems.

“You need people to break away, and then leadership might wash their hands and say, `Vote your conscience,’” the senior leadership aide says.  “There are many different power centers in the Senate. Not everything has to go through the GOP leadership.” In fairness, however, the fiscal cliff fight was easier for Dems, because all the tax cuts were set to expire — the law was on the Dems’ side. That’s not true now. “That’s the dynamic that makes this more difficult,” the aide acknowledges.

In the end, Dems are resting their hopes on the notion, as Chait has also detailed, that Republicans don’t have a clear definition of victory in this battle. In other words, if the end result they say they want is serious entitlement cuts, they aren’t going to get that until they agree to new revenues. This, combined with the possibility that the sequester cuts really could begin extracting a political toll (on both parties) over time, could ultimately force a big compromise. Dems increasingly recognize, however, that there’s a long, grueling road from here to there.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.