WASHINGTON (AP) — Former Vice President Joe Biden has raised $21.5 million since launching his White House bid in late April, his campaign said Wednesday. While Biden's haul will put him in the top tier of Democratic fundraisers, he lags behind Pete Buttigieg, a 37-year-old Indiana mayor who was virtually unknown a few months ago.

Buttigieg's campaign announced earlier in the week that he posted an eye-popping $24.8 million second-quarter haul — a stunning sum for any candidate, let alone one who is so new to the national political stage.

Biden's fundraising numbers underscore that he is a fragile front-runner. He sits atop most early polls and will have the money he needs to compete aggressively throughout the primary. But his standing as the party's elder statesman hasn't scared off his rivals, and it's clear voters are still open to other options.

Some Democratic strategists anticipated a larger fundraising number from Biden, given the connections he forged during his years as vice president and as a long-serving U.S. senator. The fact that he lagged behind expectations — and Buttigieg — will likely deepen Democratic worries that the party could be in for a prolonged primary fight at a time when President Donald Trump is making his case to voters and his reelection effort posted a massive $105 million second-quarter haul.

Still, the former vice president's supporters say they aren't worried.

"When you are the far and away front-runner, everybody is shooting at you," said Steve Westly, a Silicon Valley venture capitalist who is raising money for Biden. "One thing you have to keep in mind is this campaign is a marathon."

In addition to Biden and Buttigieg, who is the mayor of South Bend, Vermont Sen. Bernie Sanders also reported a large figure, pulling in $18 million. Yet to be seen are highly anticipated figures from Massachusetts Sen. Elizabeth Warren and California Sen. Kamala Harris, who saw a fundraising surge after her breakout performance during last week's Democratic debates .

Sanders led Democrats in fundraising during the first quarter of the year. Faiz Shakir, his campaign manager, attributed his current position further back in the pack to his rejection of high-dollar fundraisers, which Buttigieg and Biden have embraced. Instead, Sanders' campaign is fueled by small-dollar online donations from a working-class base, he said.

"It is the kind of support that we would take any day of the week over cushy, closed-door, high-dollar fundraisers in New York City, in which people eat Parmesan-encrusted salmon on a toothpick," Shakir said.

When it comes to fundraisers, Biden opens his big-dollar events to members of the media. Buttigieg does not and his campaign did not immediately respond to a request for comment Wednesday about whether that practice will change.

Biden is coming off several shaky weeks, including a dramatic moment on the debate stage during which he struggled to respond to pointed questions from Harris about his past positions on school busing.

A member of his finance team, California attorney Thomas McInerney, withdrew his support, pointing to Biden's recent comments highlighting his work with segregationists in the Senate .

Biden's campaign has tried to frame his $21.5 intake as a big win. His campaign said in an email to supporters on Monday that they "blew our fundraising goal out of the water."

In the end, though, Buttigieg packed in more fundraisers, often doing a handful of events in a day. As the first openly gay man to launch a major bid for the presidency, he also received an outpouring of financial support from the LGBT community.

Biden kept a lighter fundraising schedule.

Rufus Gifford, who was finance director of President Barack Obama's 2012 reelection campaign, praised Buttigieg's moxie.

"They are seizing on opportunities, they are building infrastructure, hiring staff and working their butts off," he said.

To be sure, fundraising success does not equate to votes, and the race is still in the early stages. Buttigieg, for example, is winning favor among donors but has struggled with black voters, a key constituency for any winning Democratic presidential campaign.

On Tuesday during a campaign appearance in Chicago, he acknowledged the need to meet more voters in the black community, suggesting they may warm to him if they "see me in action for a longer period of time."

A clearer picture of the race will emerge once the numbers are reported on July 15, laying bare the relative health of the candidates' campaigns. Candidates must reach 2% in at least four polls in addition to collecting contributions from at least 130,000 donors to appear on the September debate stage.

At least a dozen of the more than 20 campaigns are trying to lure in new donors to qualify for the fall round of debates. But the dry summer months, when fundraising is difficult, lie ahead.

Former Colorado Gov. John Hickenlooper is one of the most vulnerable candidates, in part because of his trouble raising money. He's resisting pressure from his staff to scrap his flagging presidential campaign and instead run for Senate in his home state.

Fellow Coloradoan Sen. Michael Bennet said Wednesday that he raised $2.8 million, a drop compared to the totals of his rivals.

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Associated Press writers Sara Burnett in Chicago and Nicholas Riccardi in Denver contributed to this report.