US President George Bush has threatened to veto an emergency spending bill to fund the war in Iraq because he says it has grown too expensive.
Senators have added amendments taking the total price tag to $106bn (£59bn).
The president says the bill must be cut down to size or he will refuse to sign it - something he has not done before.
The BBC's James Westhead in Washington says the threat is a sign of increased tension over spending priorities ahead of the November mid-term elections.
The Republicans' reputation for fiscal prudence has been damaged by a record budget deficit and new debt of $3 trillion since Mr Bush came to office.
Mr Bush's bill for emergency spending requests an additional $92bn - two-thirds for spending on the wars in Iraq and Afghanistan, and a third to help rebuilding in southern states after Hurricane Katrina last year.
Extra money
But senators are able to tack on additional spending provisions to such bills, and have done so. They have proposed extra spending worth a total of $14.5bn for border security, aid to farmers, measures to combat bird flu, and money to move a railway line.
"The final version of the legislation must remain focused on addressing urgent national priorities while maintaining fiscal discipline," a White House statement said.
"If the president is ultimately presented a bill that provides more than $92.2bn, exclusive of funding for the president's plan to address pandemic influenza, he will veto the bill."
Mr Bush has not vetoed a bill in more than five years as president, though in February he threatened to veto a law blocking a deal giving an Arab company control of six US ports. The company eventually ceded control of the ports to a US-owned entity.
Concern is growing among Republicans about their party's performance in the mid-term elections. According to a CNN poll, Mr Bush's approval ratings are at an all-time low of 32%.
While some Republicans have supported the spending proposals, a growing lobby have condemned them, reports our correspondent.
"We are overweight and overspending," said Representative Tom Feeney, a Republican from Florida.
To date, Congress has provided about $315bn for the Iraq war and other anti-terror spending since September 2001.
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