US Democratic presidential candidate Barack Obama raised $66m (£37m) in August, making it his best month in terms of election fundraising.
The amount raised makes it likely Mr Obama will have more to spend than Republican rival John McCain in the final two months before the vote.
Donations were lifted by half a million new donors signing up, an aide said.
The record figure contradicts suggestions that Mr Obama's fundraising appeal had been slipping.
His previous record, of $55m, was set in February.
The fundraising details are expected to be announced in the coming week when the rival campaigns file their monthly financial reports with the Federal Election Commission.
Mr Obama earlier decided not to accept public financing for the rest of his campaign and now has no spending limit.
He is the first candidate not to take public financing since the system was introduced in the mid-1970s.
Mr McCain did accept public financing, which limits his direct spending to about $84m after 1 September.
Recent opinion polls suggest Mr McCain has a lead of, on average, about 3% over Mr Obama, ahead of the 4 November vote.
Breaking the mould
Correspondents say Mr Obama raised more money than the Republican candidate partly because of the excitement generated by the Democratic nomination battle with Hillary Clinton, which ended on 7 June.
Mr McCain currently leads Mr Obama in opinion polls
Mr McCain, by contrast, wrapped up the Republican nomination back in March.
The only donations he is accepting are those to his compliance fund - money to pay for lawyers, accountants and other expenses involved in maintaining compliance with federal election laws.
The Republican National Committee, however, can still raise money to support the McCain campaign.
The Obama campaign has also broken the mould of US election finance by making big efforts to attract small donors.
Mr Obama explained his decision to shun public finance in June by saying the system was "broken".
"It's not an easy decision, and especially because I support a robust system of public financing of elections," Mr Obama said then in a video message to supporters.