White House, Senate GOP leaders reach tentative deal on fiscal crisis, sources say

White House and Senate Republican negotiators have reached a tentative deal on the fiscal crisis, sources told Fox News -- but Vice President Biden made a late-night visit to Capitol Hill to try to get rank-and-file Senate Democrats on board.

A senior official told Fox News that President Obama has gotten the sign-off from Senate Democratic Leader Harry Reid and House Democratic Leader Nancy Pelosi. But the official said they are “not spiking the ball” yet.

Both chambers of Congress still must pass whatever is introduced, and negotiators could face some heavy lifting in selling the plan to skeptical House conservatives as well as liberal Democratic senators.

The framework for a deal, though, was coming together even as it became clear Congress would likely miss a midnight deadline for action -- leaving taxpayers unsure about how much they'll be paying in 2013.

For the near-term, it appears a tax hike will technically go into effect on Jan. 1.

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But the goal is to produce a bill that could patch up the problem in the coming days, sparing most Americans from any major hit. Though the House adjourned for the night, Senate leaders were hoping to hold a vote in the coming hours.

According to a senior official, the White House and Senate Republicans reached an agreement on the last major sticking point -- what to do about the $110 billion in automatic spending cuts set to kick in starting next month.

The official said the two sides agreed to postpone the cuts by two months, in exchange for a 50-50 mix of revenue increases and spending cuts. Of those cuts, half would come from defense and half would come from other budgets.

The deal also includes an extension of current tax rates for everyone except families making above $450,000 -- up from Obama's earlier threshold of $250,000. The draft framework would also extend long-term jobless benefits for a year and address other expiring provisions like the estate tax.

Logistically speaking, it is highly unlikely that both chambers can pass anything by midnight, after which the more than $500 billion in tax hikes are set to start kicking in, followed by sweeping spending cuts. Lawmakers, though, could potentially let the tax rates lapse, only to patch up the problem in early January.

The big question is whether the plan being drafted by Senate leaders can pass both chambers -- and if so, when. The longer the stalemate drags on, the greater the risk for the economy and taxpayers.

House conservatives could take issue with the way the spending cuts were overhauled -- particularly the inclusion of additional tax increases.

On the other side, Democrats were crying foul all afternoon over the move to raise taxes only on those making over $450,000.

"Looks like a very bad deal the way this is shaping up," Sen. Tom Harkin, D-Iowa, said.

The reaction raises the possibility that if any bill is produced, Reid and House Speaker John Boehner might still struggle to sell it to their members.

But all sides were stressing the urgency of the situation.

Of the looming tax hike, President Obama said: "Middle class families can't afford it, businesses can't afford it, our economy can't afford it."

Fox News' Ed Henry, Chad Pergram and Mike Emanuel contributed to this report.