Only one caucus emerged unified from last week’s shutdown showdown, but even Senate Democrats will be challenged by the tough negotiations ahead.

The $38 billion package of budget cuts enacted last week splintered House Republicans, Senate Republicans and House Democrats — but only four members of Senate Majority Leader Harry Reid’s flock voted against the bill: Sens. Patrick Leahy (Vt.), Bernie Sanders (Vt.), Carl Levin (Mich.) and Ron Wyden (Ore.).

Democratic aides argued the Senate Democrats’ unity is a sign of just how successful the Nevada Democrat was in negotiating for his Conference’s priorities and said it will help him with the debt limit. The final deal stripped the most significant riders, such as defunding the health care law, and protected Democratic spending priorities, including health care research and Head Start.

The high-profile final fight over Planned Parenthood won by Reid helped rally liberals to the bill, while moderates were happy to be voting for cuts with the record deficit growing as an issue.

“He knows his caucus,” a Democratic leadership aide said. “He knows what they will and won’t support, what they absolutely don’t want and what they will swallow.”

Democratic Conference Vice Chairman Charles Schumer (N.Y.) even needled Speaker John Boehner (R-Ohio) on Thursday before the House voted on the package.

“Speaker Boehner needs Democrats to pass this year’s CR,” he said. “He will need them to pass a long-term deficit reduction plan as well. The sooner he abandons the tea party, the sooner we can have a compromise.”

But the Democratic unity in the floor vote and Schumer’s bravado papers over real splits within the Democratic Conference as Congress enters the more consequential debate over the debt limit and the fiscal 2012 budget.

The vote on the CR “doesn’t matter,” one Senate GOP aide said. “Are they really beating their chest going into the debt limit? Especially with [Senate Minority Leader] Mitch McConnell [R-Ky.] threatening to force Democrats to carry the water here? I don’t think Sen. Schumer should be all that confident.”

Indeed, those four Democratic defections make it technically true that Reid needed Republican votes to pass the CR in a 53-47 chamber.

Other Republicans pointed out that Reid ultimately had to give in to a far larger package of cuts than he originally sought.

“We were happy to see Democrats reject their leaders’ rhetoric that reducing Washington spending was ‘draconian’ and ‘extreme,’” McConnell spokesman Don Stewart said.

A senior House GOP aide pointed out that the original CR written by Democratic leaders, which contained only minimal cuts, flopped on the Senate floor.

“Senate Democrats are so riven and split that when they tried to write their own CR, it got fewer votes than the ‘extreme’ cuts in House Republicans’ H.R. 1,” the aide said.

Indeed, moderates have warned they want much larger cuts in the future to contain the deficit, and Democrats are divided over how to address entitlement spending and Social Security in particular.

And while the Obama administration has urged a quick and clean passage of a debt limit increase, it’s doubtful Reid could find the 51 votes that he would need without a bipartisan agreement to cut the deficit.

Already, several Senate Democrats, including Budget Chairman Kent Conrad (N.D.), have warned they will not support a large debt limit increase unless it is accompanied by a plan to contain the debt in the long term.

Sen. Joe Manchin, one of the many Democrats up in 2012, may have been the most blunt last week that the $38 billion in spending cuts won’t be nearly enough to get his vote.

“I strongly believe we must adopt a long-term, responsible and realistic fiscal plan that reflects our values and defines priorities, or I will vote against raising the debt ceiling,” the West Virginia lawmaker said. He also warned that he does not support President Barack Obama’s proposal to raise taxes as part of the solution.

“Before we ask middle-class families, or any American, for more of their hard-earned money, we need to first show them that their government can spend their money wisely,” he said. “We haven’t done that yet.”

Reid, however, also has to appeal to the liberal wing of his party. Of note, he nominated Appropriations Chairman Daniel Inouye (Hawaii) and Finance Chairman Max Baucus (Mont.) to a bipartisan negotiating group on deficit reduction proposed by the president last week.

The Baucus pick isn’t a huge surprise — his panel has jurisdiction over entitlement spending and would have to move any package — but he has been frosty to the idea of the deep entitlement cuts envisioned by the president’s fiscal commission. Baucus served on the president’s deficit commission last year and vigorously opposed the proposal put forward by the chairmen to save $4 trillion over the coming decade.

Baucus this week suggested instead attaching some kind of deficit caps to a debt limit increase — an idea that also found its way into Obama’s deficit speech Wednesday. But Obama’s proposal would force tax increases as well as spending cuts.

Republican proposals focus exclusively on spending.

They intend to force Democrats into politically tough votes on a balanced budget amendment to the Constitution as well as spending caps, including a proposal by Sens. Claire McCaskill (D-Mo.) and Bob Corker (R-Tenn.) that would slash spending by $7 trillion over the next decade — more even than the House Republican budget.

Reid’s biggest challenge, if he chooses to accept it, could be to bring a fiscal 2012 budget to the floor. He declined to do so last year in what has become an endlessly repeated GOP talking point, but a budget resolution would pose special challenges because there are no limits on amendments and most can pass with a simple majority. For Reid, who has a slim majority and a passel of nervous Democrats staring at re-election next year, fighting off GOP amendments won’t be easy, and many Republicans doubt he will bring up a budget for that very reason.