Citing news reports, the Republican National Committee said Mr Obama had accepted contributions from foreigners and taken more than the US$2,300 (S$3,600) maximum from donors who give in small increments. The Obama campaign denied the charges.
The RNC said it will ask the Federal Election Commission to examine Mr Obama records in detail to determine the extent of the problem.
The Obama campaign could face fines if found guilty of violations by the FEC, but any decision would likely come after he faces Republican John McCain in the Nov 4 presidential election.
Obama spokesman Bill Burton said the McCain campaign has had to return over US$1.2 million to donors who potentially violated the law with their contributions, including money from foreign nationals.
'Our campaign has shattered fund-raising records with donations from more than 2.5 million Americans. We have gone above and beyond the transparency requirements,' Mr Burton said.
'While no organisation is completely protected from Internet fraud, we will continue to review our fund-raising procedures to ensure that we are taking every available to step to root-out improper contributions,' he said.
But Republican officials said the Obama campaign had not done enough to weed out illegal donations.
'It seems to the RNC that the Obama campaign knew they were excessive,' RNC chief counsel Sean Cairncross said in a conference call. 'Yet they appear to have taken no action on their own.'
Mr Obama opted out of the public financing system so his money totals include both the primaries and the general election.
More than half of the US$454 million raised by Mr Obama has come in small increments of US$200 or less.
By contrast, one-third of Mr McCain's US$230 million raised during the primary campaign has come in small donations.
Mr McCain is taking public funds in the general election campaign so he is limited to US$84 million.
Campaigns are not required to report small donations, and some donors appear to have given well beyond the legal limit, Newsweek magazine reported.
Two apparently fictional donors using the names 'Doodad Pro' and 'Good Will' gave Mr Obama more than US$11,000 in increments of US$10 and US$25, according to Newsweek.
Other news accounts suggest that roughly 11,500 donors who gave a total of US$34 million to the campaign may be citizens of foreign countries, who are not allowed to contribute to US elections, the RNC said.
'We see a lack of control, a lack of willingness on the part of the Obama campaign to ask relevant questions,' Mr Cairncross said. -- REUTERS