Mitt Romney and the Republicans raised a combined $100m (£64m) in June, setting a one-month record for any of the party’s campaigns, US media report.
The figure excludes the millions raised by groups that support the Republican presidential candidate.
President Barack Obama and the Democratic Party have not yet released their fundraising figures for June.
Mr Obama spent the day in Ohio as he launched the first bus tour of his campaign for re-election in November.
In May, the Romney campaign out-fundraised his rival, attracting $77m against the Obama campaign’s $60m.
Mr Obama has been warning supporters that he is in danger of becoming the first sitting president in history to be outspent by his opponent.
According to Mr Romney’s campaign aides, much of the money raised in June came from new donors, Politico reported.
Several states, including the key battlegrounds of Colorado, Michigan and Ohio exceeded their fundraising goals, they added.
Meanwhile, Mr Obama pitched a positive message on Ohio’s economic recovery and the comeback of the state’s car industry.
Ohio’s unemployment rate of 7.3% is well below the national average of 8.2%.
Ohio is a key battleground in presidential elections – no Republican has ever won the White House without capturing it.
A Quinnipiac University poll of Ohio voters last week had Mr Obama leading his rival by 47% to 38% in the state, following a barrage of attacks on Mr Romney’s business record.
“I’m betting you’re not going to lose interest,” Mr Obama told voters in Maumee, Ohio. “I’m betting you’re not going to lose heart. I still believe in you, I’m betting on you.”
The president also touched on his landmark healthcare reform act, most of which was upheld by the Supreme Court last week.
“It is going to make the vast majority of Americans more secure,” he told supporters. Mr Romney has pledged to repeal the law if he wins office.
The former Massachusetts governor, who is on holiday with his family in the state of New Hampshire, said the president had “no new answers” for the economy.
On Friday, Mr Obama will finish his bus tour with appearances in Poland, Ohio, and Pittsburgh, Pennsylvania.
With the economy such a major issue in the coming election, both presidential campaigns will be eagerly awaiting figures to be released on Friday showing whether there was any rise in hiring by US employers during June.