At a Martha's Vineyard fundraiser Monday night, President Obama warned Democrats not to take this presidential election lightly and to be aggressive in their campaigning.

"If we are not running scared until the day after the election, we are going to be making a grave mistake," Mr. Obama said at the event for the Hillary Victory Fund, which raises money for Clinton's campaign, the Democratic National Committee and state parties.

"We are still going to have to fight what has been an unrelenting negative campaign against her that has made a dent in the opinion of people even who are inclined to vote for her," he added. "We've got be aggressive in our campaign for 80 days, and that's not a lot."

There are 83 days until Election Day on Nov. 8.

Though he went after the GOP nominee, the president said he's weary of the topic of Trump.

"Frankly I'm tired of talking about her opponent," Mr. Obama said. "I don't have to make the case against her opponent because every time he talks, he makes the case against his own candidacy."

He praised Clinton's leadership, and said she would be a "capable, visionary, hardworking, diligent, smart, tenacious leader in the Oval Office."

"When I say that she knows what she's talking about, it's because I have seen her do the work," he said.

Mr. Obama has been vacationing on Martha's Vineyard with the first family since last week and will return to Washington next week.