Top Republican John Boehner and leading Democrat Harry Reid are tweaking their budget plans after nonpartisan analysts said their sums did not add up.
The US Congressional Budget Office (CBO) said neither Mr Boehner's plan nor Mr Reid's would cut the deficit by as much as they had both estimated.
House Republicans have been meeting behind closed doors to discuss the Boehner plan.
Mr Reid said Mr Boehner's proposal could not pass the Senate.
The US runs a budget deficit that topped $1.5tn (£920bn) this year, and has amassed a national debt of $14.3tn.
Lawmakers have been locked in weeks of bitter negotiations over raising the US borrowing limit by the deadline of 2 August.
The US treasury department has said it will no longer be able to borrow money on that date, and analysts say soon after that it will be unable to pay all of its bills.
In recent decades, the US Congress has increased the government's borrowing authority dozens of times as a matter of routine, but this year, conservative Republicans have demanded steep cuts to the budget deficit as the price of an increase.
The bill proposed by Mr Boehner was being rewritten on Wednesday after the CBO's analysis found it would cut spending less than had been advertised - about $850bn over 10 years, rather than $1.2tn.
His plan would also raise the debt limit only for a short term, ensuring another debt limit fight between now and the 2012 elections.
Republicans planned a House vote for Thursday on the reworked plan.
Mr Reid's office told the BBC the Senate majority leader was also revising his plan after the CBO said the proposal would cut $2.2tn from deficits over the next decade, about $500bn less than had been claimed.
His bill would raise the debt ceiling by $2.7tn.
Mr Reid has not said when his bill may reach the Senate for a vote.
The White House warned on Wednesday that time was quickly running out to reach a compromise on the US debt limit.
"That deadline is hard and fast," White House spokesman Jay Carney said of next Tuesday's expiry date on the debt ceiling.
"People keep looking for off-ramps. They don't exist," he added.
Reports suggested Republicans were beginning to coalesce around Mr Boehner's plan, even though the Democrats who control the Senate have rejected it and the White House has said President Barack Obama's senior advisers would recommend he veto it.
Mr Reid said the Boehner plan was "a big wet kiss for the right wing", and Mr Obama has said he will not sign a short-term deal.
Budget demand 'bizarro'
Mr Carney criticised Mr Boehner for scheduling a Thursday vote on his plan despite the White House's express rejection.
"Why are we voting on measures that have no chance of becoming law?" he asked during a news conference.
Also, in a new twist that analysts say illustrates the discord among the Republican party, Republican Senator John McCain, the party's 2008 presidential nominee, took to the Senate floor to criticise House Republicans who continue to push for a balanced budget amendment to the US constitution as the price of raising the debt ceiling.
A balanced budget amendment - which would require the US government to hold down spending to a certain percentage of gross domestic product and run in the black - was already rejected by the Senate and by Mr Obama.
Mr McCain called the demands "foolish" and "bizarro" and said they were "deceiving many of our constituents" and blamed them on newly elected House members' inexperience.