US President Barack Obama has thrown $US5000 ($A4786) of his own cash into his campaign pot as he pleaded with supporters to help him reverse opponent Mitt Romney's fundraising advantage.
The Republican challenger raised more than the Democratic incumbent in both May and June and after Obama making his personal donation he sent out a mail urging backers to get behind him on the last day of July fundraising.
Obama's email was directed at those small-scale donors who might offer as little as three dollars.
"On it's own, what I gave won't be enough to surmount the unprecedented fundraising we've seen on the other side," he warned.
"But we have always believed that there's nothing we can't do when we all pitch in," he added.
The monthly fundraising tally is seen as an important indicator of a candidate's overall support and a deep treasure chest is crucial to keeping up the onslaught of television advertising as the November 6 election approaches.
The Democrats suffered setbacks in May and June, when Romney raised significantly more money. The multi-millionaire businessman's campaign raked in more than $US106 million in June alone, $US35 million more than Obama.
June's numbers were also significant, because they showed a dramatic increase in grass-roots donations for Romney, turf Obama had broadly dominated.
In a separate email, the president said on Tuesday was "one of the most critical fundraising deadlines we'll face", and winning the election will depend on "what you did in moments like this to close the gap".
But Obama's team is also seeking out larger donors and on Monday evening he collected $US2.4 million at a $US40,000 a plate campaign dinner in New York.
For weeks, Obama and the organisations that support him have been pushing for cash, all while insisting they will surely raise less money than their competitors, which they say is a first for the incumbent.
Its a turnaround from Obama's first presidential run in 2008, when his fundraising success was striking. In September 2008, his campaign raised $US150 million, the record for the highest amount ever raised in a single month.