US President Barack Obama has outraised Republican opponent Mitt Romney for the past month, the first time for the incumbent in four months, according to campaign announcements on Monday.

Obama's campaign manager Jim Messina said the campaign raised 114 million dollars in August, while the Romney campaign announced a total of 111.6 million.

"The key to fighting back against the special interests writing limitless checks to support Mitt Romney is growing our donor base, and we did substantially in the month of August," said Messina in a statement. "Fueled by contributions from more than 1.1 million Americans donating an average of 58 dollars - more than 317,000 who had never contributed to the campaign before - we raised a total of more than 114 million dollars."

Before the August numbers came out, the Obama campaign had been outraised by Romney for three months in a row, raising questions for the president's reelection campaign.

The fundraising number for Romney from May to June were 77 million, 106 million and 101 million, while Obama raised 61 million, 71 million and 75 million.

The Obama campaign did not disclose the amount of money they had on hand. Republicans said Monday that they entered September with 168.5 million in the bank.

Obama has entered September not only with a fundraising advantage, polls have shown he garnered more support from his party's national convention last week than Romney had from the Republican National Convention late August.

According to Gallup's seven-day rolling average for the past week, released on Sunday, 49 percent of registered voters say they will choose Obama if the election were held today, and 45 percent said they would choose Romney. A Reuters/Ipsos poll also showed 47 percent of voters say they would vote for Obama, compared to Romney's 43 percent.

However, fundraising numbers from outside groups showed conservative Super-PACs like American Crossroads and Restore Our Future, which support Romney, have substantially outraised groups supporting Democrats this cycle.