WASHINGTON, Aug. 1, 2011 -- President Obama and top congressional leaders breathed a collective sigh of relief Sunday night after reaching a tentative deal to raise the nation's $14.3 trillion debt limit and avert U.S. default. But now they have to make it official.

The House is expected to vote on the plan later tonight and, assuming it passes, the Senate could consider it tomorrow.

But House Republican leaders are getting increasingly nervous about whether the debt deal will pass because they fear there may not be enough Democratic yes votes to get them over the top.

A top GOP House leadership aide told ABC News that if Democrats are not actively trying to get votes, "then we should be nervous ... I hope the White House has a contingency plan, like sending the President up here to plead for votes."

It is clear that both sides will have to find votes for the package. Up to 100 Republicans could vote against their party, which means House Minority Leader Nancy Pelosi will have to deliver a good portion of the Democratic caucus votes -- up to 75 -- for a bill that has gotten tepid response at best. Many Democrats had pledged not to vote for a bill that relied solely on spending cuts and did not off-set them with tax hikes. This bill does not.

"As my son would say, we just got to suck it up, buttercup," said Sen. Mark Begich, D-Alaska, of the need to support the bill. "At the end of the day, we're going to do it, because it is the right thing to move forward. It's going to be tough, and we'll get criticism for things we could have done, but we are where we are and we need to move forward."

In the House, the math could be much more difficult as leaders from both parties tried to put the onus for finding votes on the other.

"Republicans I presume are going to have to get the votes," said Rep. Steny Hoyer of Maryland, the Democrats' top vote counter.

After meeting with his Republican colleagues, House Speaker John Boehner was insistent that the deal, however, imperfect, should be passed.

"It gives us the best shot we've had in the 20 years I've been here to build support for a balanced budget amendment to the Constitution to put the fiscal handcuffs on this Congress that are sorely needed," Boehner said.

House Majority Leader Eric Cantor, Boehner's chief deputy, sought to mollify Republican members frustrated at the level of spending cuts by comparing changing government spending to turning around an aircraft carrier -- it doesn't happen immediately.

And while many Republicans have already said they would oppose the bill, they are not actively opposing the deal.

Rep. Jason Chaffetz, R-Utah, said on ABC News' "Top Line" that he would vote no, even though the process has been a win for Republicans.

"The fact that we're now having a serious discussion about debts where we're talking about cutting spending, that there's no tax increases that we're talking about right now, that's a huge -- at least moral -- victory, I think for a lot of fiscal conservatives that were concerned about the financial health of this country," he said.

The compromise's fine print has plenty of critics on both sides of the aisle, leaving leaders of both parties scrambling to sell the package to their respective members and secure the votes they need to get it passed.

The nonpartisan Congressional Budget Office released a report this morning suggesting the debt framework would trim government spending by $2.1 trillion over 10 years.

Many Democrats are angry that the spending cuts are not coupled with tax increases, as Obama had promised in seeking a "balanced approach," while others have decried a lack of immunity for social service programs from spending cuts.

"This is a Satan sandwich," the head of the Congressional Black Caucus, Rep. Emanuel Cleaver, D-Mo., said in an interview with ABC News. "There's no question about it because there's nothing inside this sandwich that the major religions of the world would say deals with protection for the poor, the widows, the children. It's not in here."

The White House dispatched Vice President Joe Biden to Capitol Hill to meet with House and Senate Democrats to lobby for their support.

"Is this the deal I would have preferred? No," Obama said at the White House last night. "But this compromise does make a serious down payment on the deficit reduction we need, and gives each party a strong incentive to get a balanced plan done before the end of the year."

Meanwhile, many Republicans have also voiced disapproval of the deal, some opposing any increase in the debt limit altogether, with others pledging only to support it if a balanced budget amendment to the Constitution is passed first.

"It isn't the greatest deal in the world, but it shows how much we've changed the terms of the debate in this town," Boehner said on a Sunday night conference call with House Republicans, according to GOP officials. He urged his colleagues to support the deal because it's "all spending cuts. The White House bid to raise taxes has been shut down."

The compromise legislation would increase the government's borrowing power by up to $2.4 trillion through 2013, and impose nearly $1 trillion in spending cuts in 10 years, most coming out of the non-defense discretionary budget.

A new special congressional committee (nicknamed the "super committee") would be required to recommend additional deficit reduction -- that would likely include changes to entitlement programs and the tax code -- of up to $1.5 trillion by Thanksgiving. Under the deal, Congress would have to pass the recommendations into law by December or face the "trigger" of stiff, automatic cuts.

"We think that at the end of the day, this is an agreement that will pass the Senate and the House and President Obama will sign it," White House senior adviser David Plouffe said.