WASHINGTON -- With crisis concerns rising, House Republican leaders shrugged off a White House veto threat and an outbreak of tensions within their own party Wednesday as they built support for legislation to stave off the government default threatened for next week.

Worried Wall Street sent stocks plunging on fears that political gridlock would prevail.

"I can't do this job unless you're behind me," House Speaker John Boehner bluntly told his fractious rank and file in the run-up to a scheduled vote today on the bill, which was rewritten to show deeper spending cuts than 24 hours earlier.

With Boehner facing a major test of his leadership, the While House disparaged the measure he was working so hard to pass. Sen. Harry Reid of Nevada called it "a big wet kiss for the right wing," and all 51 Senate Democrats and two independents pledged to scuttle it if it cleared the House.

The White House has threatened a veto, saying the bill does not meet President Barack Obama's demand for an increase in the debt limit large enough to prevent a rerun of the current crisis next year.

Instead, Obama supports an alternative drafted by Reid that also cuts spending, yet provides enough additional borrowing authority to tide the government over through next year.

For all the bluster, there were hints that a compromise might be near.

"Magic things can happen here in Congress in a very short period of time under the right circumstances," said Reid, the Senate majority leader.

Without legislation in place by Tuesday, administration officials have said the Treasury will not be able to pay all the nation's bills, possibly triggering a default that could prove catastrophic for an economy still recovering from the worst recession in decades.

The U.S. financial markets posted big losses for the day as political leaders maneuvered. Some issued an online call for Boehner to resign as speaker, while others said he deserved time to try to strike the best deal possible.

The Republican legislation underwent revisions to increase its prospects of passage.

That meant changes that brought projected savings for 2012 to $22 billion, part of a 10-year cut of $917 billion in all that would trigger a $900 billion increase in the debt limit. The bill also would establish a special committee of lawmakers to recommend additional cuts that would trigger additional borrowing authority if approved.

While the two parties' bills differed in key details, they also shared similarities that underscored the concessions made by both sides in recent days.

Reid's bill does not envision a tax increase to reduce deficits, a bow to Republicans. But neither does the House measure require both houses to approve a constitutional balanced budget amendment for state ratification, a step in the direction of Obama and the Democrats.

For Boehner, the House vote shaped up as a critical test of his ability to lead a majority that includes 87 first-term lawmakers, many of them elected with tea party support.