WASHINGTON — President Barack Obama’s campaign raised more than $70 million combined for his re-election and the Democratic party during the summer, an amount that gives him a clear advantage over his Republican rivals but is less than his initial fundraising effort.

The fundraising total announced Thursday exceeds a goal set by the campaign of $55 million combined for the July-September fundraising period but is about $16 million less than Obama raised during the April-June quarter.

Obama has dealt with declining poll numbers and a weakened economy during the summer, prompting the president to recently call himself the “underdog” in the presidential race. Campaign officials had said they would raise less because of canceled fundraisers during the summer’s debt ceiling negotiations and a typical summertime lull in raising cash.

Obama campaign manager Jim Messina said in an e-mail to supporters that more than 600,000 people donated to the campaign in the most recent quarter, more than the previous three months. He said more than 980,000 people have given money to the campaign, and in the most recent quarter, 98 percent of the donors gave $250 or less, with an average donation of $56.

“Getting to a million grassroots donors isn’t just a huge accomplishment this early in the campaign. It’s our answer to our opponents, the press, and anyone who wants to know whether the President’s supporters have his back,” Messina said.

The numbers include $42.8 million for Obama’s campaign and $27.3 million for the Democratic National Committee, which will help Obama’s re-election effort next year.

Obama still leads his Republican rivals in fundraising by tens of millions and can save most of it for next year because he does not face a primary opponent.

Former Massachusetts Gov. Mitt Romney leads the Republican field in fundraising, pulling in more than $18 million in his first three months of campaigning. He is not expected to surpass that mark for the past three months but should lead the pack in cash on hand. Texas Gov. Rick Perry raised $17 million in his first seven weeks of campaigning and had $15 million in the bank.

Obama advisers have told donors privately they hope to match or exceed the $750 million they raised in 2008, a staggering amount that would help the president pay for expensive TV ads and a massive get-out-the-vote operation. But to match the amount they raised the last time, Obama would need to bring in nearly $120 million combined for each of the next five quarters to keep pace.

Messina said the campaign was using the money to build its operation. He said the campaign has opened up three new field offices every week during the past three months, and volunteers and organizers have made 3 million phone calls and in-person visits to voters.

“We’re up against a Republican Party and special interest-funded groups that will spend hundreds of millions of dollars spreading any message that they believe will defeat the President and roll back our efforts to build a fairer economy that rewards hard work and responsibility, not large corporations,” Messina said.