Mr. Obama’s heavy expenditures — and his campaign’s pressure on bundlers to find and groom new donors — have stirred worries among other Democrats, who have long taken Mr. Obama’s financial supremacy for granted.

“There is a lot of worry that Romney’s folks are raising so much more,” said one of Mr. Obama’s top fund-raisers, who did not want to be identified as discussing internal campaign business. “I just don’t think there’s a lot of high-dollar money left on the table.”

In fact, Republicans insist that Mr. Obama will rue his spending.

“Heading into the final laps of the campaign, the Democrats will regret squandering so much of their haul early in the cycle on massive monthly overhead,” said Sean M. Spicer, a spokesman for the R.N.C.

But in interviews, party and campaign officials defended the approach of spending money to build out the campaign, saying they believed that the wisdom of Mr. Obama’s strategy would be demonstrated at the voting booth in November.

“The earlier the better,” said Adam Fetcher, an Obama campaign spokesman. “Starting a conversation with a persuadable voter months before Election Day allows us to be more effective in responding to that voter’s priorities than if they first hear from us a few weeks out. Building and maintaining our grass-roots foundation takes time and resources, but we believe those early investments will make a difference.”

But grass-roots movements do not come cheap.

Through June 30, Mr. Obama and the Democratic National Committee had spent $46 million on direct mail and postage, according to F.E.C. records. Legal fees added up to $3 million, while $25,000 went to flower arrangements. Phones and telemarketing have eaten up at least $24 million, and Internet advertising $36 million, part of a sophisticated effort to try out different fund-raising appeals, test attacks on Mr. Romney and reach small donors. The campaign reached two million total donors in May, a campaign official said, a tally it did not reach until August during the 2008 election cycle.

Mindful that the recession has displaced many people who voted for Mr. Obama in 2008, especially those with low incomes, the campaign has also invested heavily in voter registration. That has paid dividends in states like Nevada, where Democrats have steadily expanded their registration advantage over Republicans in recent months. In Ohio, the early deployment of money and a field staff last year also allowed the campaign to help Democrats fight a Republican-led effort to restrict early voting in the state.