House and Senate negotiators on Tuesday unveiled a modest $85 billion budget agreement that would fund the government through most of 2015, replace some of the sequester-mandated cuts, and avert another government shutdown.

But don't go throwing the confetti just yet. Though the deal has the bipartisan backing of congressional leaders and President Obama, it has quickly rankled conservatives who are furious that it would raise spending above the $1 trillion mark. And some Democrats, too, are displeased with the deal since it wouldn't extend unemployment benefits.

With Congress on a holiday-shortened schedule, there is little time for lawmakers to act. And given that the hyper-partisan Congress triggered a rare government shutdown mere moths ago over fiscal differences, the deal's passage is hardly a sure thing.

Under the deal, discretionary spending would be set at $1.012 trillion next year, or $45 billion higher than it would otherwise have been. And the deal would undo $65 billion in sequester cuts, with the relief split evenly between defense and non-defense spending.

Both Rep. Paul Ryan (R-Wis.) and Sen. Patty Murray (D-Wash.) predicted the deal would pass, as did key leaders from both parties. And Obama urged its passage, calling it a "step in the right direction."

"This agreement doesn't include everything I'd like — and I know many Republicans feel the same way" President Obama said in a statement. "That's the nature of compromise."

Getting party leaders behind the deal is one thing. Getting skeptical — or, in some cases, downright furious — rank and file members is another.

"I don't think anything is a sure deal in the House or the Senate," Sen. Pete Sessions (R-Ala.) told Politico. "It's not going to be a happy experience for most members, liberals, and conservatives."

On the GOP side, conservative members — who in October dug in on the government shutdown — panned the deal for raising spending and upping federal fees, which they said were no different than tax increases.

Likewise, Sen. Marco Rubio (R-Fla.), a possible presidential contender who endorsed the last shutdown, said the deal "continues Washington's irresponsible budgeting decisions by spending more money than the government takes in," and that the American people "deserve better."

Thirty conservative House members on Tuesday sent a letter to Speaker Boehner (R-Ohio) calling for spending to continue as planned: "The Budget Control Act is the law of the land," read the letter. "Our Democrat colleagues are now threatening to shut the government down in order to change that. We should not permit that to happen."

Some influential conservative groups whose organizing helped spark the last shutdown — Heritage Action, FreedomWorks, and Americans for Prosperity — also came out against the deal. FreedomWorks announced they vociferously oppose the agreement, while Heritage called it a "gimmicky, spend-now-cut-later deal will take our nation in the wrong direction."

On the Democratic side, some lawmakers were miffed that the deal would not extend unemployment benefits, which are set to expire on December 28. Democrats have made a strong push for such an extension in recent days, with House Minority Leader Nancy Pelosi (D-Calif.) suggesting, then walking back, a red line that a budget deal would be DOA in the House without such a provision.

On Wednesday, Pelosi said she was unsure how the House Democratic caucus would align on the deal.

In October, Boehner had to rely on near-unanimous Democratic support in the House to advance a bill to end the shutdown. If his caucus' right wing revolts again, and enough Democrats also buck the deal, the agreement would face a very narrow path to passage.

Both parties, especially the GOP, would love to avoid another bruising shutdown. But whether even this small agreement can survive the legislative gauntlet is an open question.