A Republican deficit reduction plan headed for two major votes in Congress on Thursday and its expected demise could force a compromise to avert an imminent and unprecedented debt default.

But the vote in the Republican-controlled House of Representatives was delayed until later at night as that party's leadership struggled to muster support for its plan.

A Democratic aide said Republicans were two to four votes short, raising fresh doubts over the ability of Congress to bridge a deep partisan divide.

World markets, unnerved by the risk of a U.S. default or credit downgrade, were watching anxiously. The U.S. stock market's broad S&P 500 index fell for a fourth day and interest rates soared on some Treasury bills that mature in August.

Despite the gridlock, Congress could kick into higher gear as pressure to reach a deal mounts ahead of next Tuesday, when the White House says the government will run out of money to pay its bills.

Top U.S. financial executives added their voices to calls from the business community for Congress to make a deal that would banish the specter of default.

The priority of President Barack Obama, a Democrat, was to "lift the cloud and make sure that the United States does not default," White House spokesman Jay Carney told reporters, urging Congress to end its battle and deliver a compromise.

It was unclear whether the deficit reduction bill presented by House Speaker John Boehner, the top Republican in Congress, would overcome objections from conservative rebels in his own party.

The vote was originally scheduled for between 5:45 p.m. and 6:15 p.m. EDT. A Republican aide said the House would vote on the bill later on Thursday evening but gave no specific time.

Republican leaders had been engaged in a final round of arm-twisting as they tried to secure the magic number of 217 votes needed to pass the bill in the House and avoid a humiliating defeat.

The Democratic-controlled Senate plans to vote on the Boehner bill afterward and it would be defeated in that chamber, Senate Majority leader Harry Reid said.

A failure to raise the debt limit by August 2 could trigger a payments crunch that would shake the global financial system and could tip the United States back into recession.