Mitt Romney and the Republicans raised a combined $100 million in June, laying down the gauntlet to a Barack Obama campaign that is scrambling to keep pace.
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 results for June.
Barack 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 its rival, attracting $77 million against the Obama team’s $60 million.
Barack 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 Mitt 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, Barack 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% compares with a 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 Barack Obama leading his rival by 47% to 38% in the state. Earlier, the Obama campaign had run a barrage of attacks on Mitt Romney’s business record.
“I’m betting you’re not going to lose interest,” Barack 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, which was upheld by the Supreme Court last week.
“It is going to make the vast majority of Americans more secure,” he told supporters. Mitt Romney has pledged to repeal the law if he wins office.
On Friday, Barack Obama will finish his bus tour with appearances in Poland, Ohio, and Pittsburgh, Pennsylvania.
Mitt Romney, a former Massachusetts governor, said the president had “no new answers” for the economy.
Despite its fundraising bonanza, correspondents say the Romney campaign has struggled recently to gain the initiative.
His team was seen as having been put on the back foot by the president’s order last month halting deportations for children of illegal immigrants.
The Romney camp also delivered conflicting responses to Supreme Court ruling last week on healthcare.
And there were negative headlines this week over a Vanity Fair investigation reporting that much of Mitt Romney’s personal fortune was hidden in a network of opaque offshore investment havens.
Conservative concerns over the former private equity chief’s campaign were vented in an editorial in the Wall Street Journal on Wednesday.
It accused Mitt Romney’s staff of “slowly squandering an historic opportunity”. The newspaper is owned by Rupert Murdoch, who called for a Romney campaign shake-up, in a tweet earlier this week.
Meanwhile, pictures of Mitt Romney on holiday with his family this week in New Hampshire prompted conservative radio talk host Laura Ingraham to tell listeners:
“There’s no week to spare, we have a country to save.”
With the economy such a major issue in the coming election, both campaigns will be eagerly awaiting figures due on Friday showing if there was any rise in hiring by US employers during June.