Mitt Romney and the Republicans have raised $101m (£65m) in July, exceeding President Barack Obama's fundraising total for the third straight month.
Mr Obama and the Democrats raised $75m last month.
Opinion polls show the presidential candidates are locked in a tight race, with Mr Obama marginally in the lead.
With exactly three months to November's election, Mr Obama could become the first sitting president to be outspent in the final leg of the campaign.
His fundraising prowess during his 2008 campaign for the White House was unrivalled.
Party conventions loom
But this time, Mr Obama faces independent fundraising groups known as Super Political Action Committees (Pacs) that can spend unlimited amounts of money backing presidential candidates and other political causes.
Republican-backed Super Pacs have raised much larger amounts of money than similar, pro-Democrat groups.
This presidential election is expected to be the most expensive in US history.
In July, more than 94% of Mr Romney's donations were $250 or less, totalling $25.7m.
But 6% of Romney donors accounted for 75% of the funds raised by him last month.
Meanwhile, the Obama campaign said 98% of its donations in July were under $250, although it did not specify how much money they accounted for. The average contribution to the campaign was $54.
As the details of the candidates' campaign finances were revealed, Mr Obama was set to attend two fundraising events in the state of Connecticut.
One event will be held at the home of Hollywood tycoon Harvey Weinstein. Actress Anne Hathaway and screenwriter Aaron Sorkin were expected to be among the guests.
Mr Romney is not campaigning on Monday while he attends meetings at his holiday home in the state of New Hampshire.
On Saturday, the former governor of Massachusetts is scheduled to begin a four-day bus tour that will pass through the states of Virginia, North Carolina and Florida before finishing in Ohio.
All four states are seen as key battlegrounds that could be carried by either candidate in November.
Recent figures suggest Mr Romney, the Republican National Committee and state Republican parties have almost $186m in cash on hand.
Similar figures for Mr Obama's campaign have not been disclosed, but the Democrats are thought to be ahead in overall fundraising.
They have raised about $627m, while the Republicans have raised some $495m.
Both candidates are entering the final stretch of fundraising before they can begin spending their warchests.
Correspondents say the rivals are investing heavily in campaign infrastructure - new staff and local offices - to help rally grassroots support.
US electoral finance laws say campaigns can accept up to $5,000 from a single donor, but can only spend half that amount before the conventions.
Mr Romney is to be formally anointed as his party's presidential nominee at the Republican National Convention in Tampa, Florida, later this month.
The Democratic National Convention takes place in Charlotte, North Carolina, in the first week of September.