Republican presidential candidate Rudy Giuliani has raised $11m (£5.5m) in the past three months, his campaign says, $1m more than rival Mitt Romney.
The key contenders are (from L) McCain, Giuliani and Romney
Mr Romney, a former Massachusetts governor, also lent $8.5m to his own campaign. Arizona Senator John McCain will report raising $6m, aides said.
Ron Paul, an anti-war congressman from Texas who has a big internet following but is low in the polls, raised $5.1m.
However, Democratic rivals raised much higher amounts over the same period.
Frontrunners Hillary Clinton and Barack Obama raised $27m and $20m respectively, their campaigns reported.
The third quarter of the year is traditionally seen as a difficult period in terms of fundraising because it falls over the summer months when many people are on holiday.
But the candidates will now be keen to have as much in the bank as possible as they approach the primary elections starting in January.
Primaries are held to decide which candidate will represent each party in the full presidential election in November 2008.
Mr Giuliani, a former New York mayor, has a reported $16m in reserve for the final push to January's vote.
Mr Giuliani's high fundraising total was a sign that Republicans see him as the candidate most likely to win in November 2008, his campaign manager said.
Mr Romney's latest contribution means he has now put $17.5m of his own money into his campaign this year. He has $9m left in hand for the remaining months.
He reported reaching 23,000 new donors in the third financial quarter, giving him a donor base of about 100,000.
"Our campaign made considerable progress this quarter, expanding Governor Romney's support across the country," said his spokesman, Kevin Madden.
Both candidates saw their fundraising fall from the second quarter, but still outpaced their Republican rivals.
Senator McCain's campaign manager said that despite a drop in funds his candidate's campaign was still on sound financial footing - and gaining momentum in states that vote early in the primary season.
The amount raised by Mr Paul, who has generated a strong buzz on social networking sites like MySpace and Facebook, was more than double his second quarter tally.
Former Tennessee Senator Fred Thompson, who only formally announced his candidacy in early September, is expected to report raising over $8m in the past quarter.
All the candidates must officially report cash raised from July to September to the US Federal Election Commission by 15 October.