Romney's general election campaign hopes to raise $600 million and Obama's is expected to raise at least $750 million, The New York Times reported.
Republican-leaning groups and Democratic-leaning unions plan to spend hundreds of millions more, the newspaper said.
"This is going to be the most moneyed election in the history of the United States," said Bob Edgar, president of Common Cause, which supports more restrictions on campaign spending.
Edgar, a former congressman elected after the Watergate scandal, said the current campaign brings a "sense of coming full circle, of forgetting our history -- the reason we installed a system for financing campaigns that didn't rely on corporate or wealthy money."
Obama decided against receiving public funds in 2008, when he raised $750 million, and in so doing broke a campaign promise, then spent four times as much as Republican nominee John McCain.
The president has had more than 100 fundraisers for his campaign and raised hefty amounts jointly with the Democratic National Committee.
Romney, the former Massachusetts governor, plans dozens of fundraisers between now and the end of June.