WASHINGTON — Former Vice President Joe Biden announced raising $15.2 million in the third quarter of the year, behind some rivals and short of what he raised in the previous quarter of the year.

Biden, a leading 2020 contender, has suffered shrinking poll numbers in recent weeks and the fundraising numbers announced Thursday may further fuel questions about whether he can maintain the sense of inevitably his campaign has sought to create around his candidacy.

The third quarter of the year, which spans from July to the end of September, is typically the slowest of the year, but Biden was outflanked by at least two opponents.

Sen. Bernie Sanders, I-Vt., tapped his small-dollar fundraising army for an impressive $25.3 million haul in the quarter, while Mayor Pete Buttigieg of South Bend, Indiana, who has become a darling of the left-leaning donor class, posted a $19.1 million haul.

California Sen. Kamala Harris raised less than the former vice president at $11.6 million.

Massachusetts Sen. Elizabeth Warren, who has been gaining on Biden in polls, has yet to announce her fundraising.

Candidates have until Oct. 15 to file reports with the Federal Election Commission.

Biden personally revealed the numbers during a fundraiser in Palo Alto, Calif, on Thursday, telling donors they were among the more than a half-million individuals who have given to his campaign.

Biden aides had been mum all week about when it planned to release his third-quarter haul.

"Today’s fundraising totals put the campaign in a strong position as we enter the fall," said Biden campaign manager Greg Schultz in a statement. "The question any campaign faces at this point is whether or not you have the resources to compete in early states and sustain your efforts beyond. Our campaign unequivocally does and builds on our strength each week."

The campaign announced a $6 million ad buy earlier Thursday, Schultz noted, and he said the campaign has "well over 200 staff and dozens of field offices" in key states.

Buttigieg was also the money leader in the spring, but Biden was not far behind. He had entered the race late, giving him less time to raise money in the second quarter.

In July, Biden's campaign announced it had raised $21.5 million since its launch in late-April, while Buttigieg raised $24.8 million.

The drop-off of more than $6 million from Biden's initial total to now comes even as he made time for more big-dollar fundraisers in the last three months.

According to an NBC News tally, Biden attended 24 fundraisers in the second quarter and 43 in the third. Biden was attending a pair of fundraisers on Thursday in the Bay Area.

It was not immediately clear whether the higher number of big-dollar events was an effort to compensate for a possible downturn in small-dollar, online giving. In July, the campaign said it had received 436,000 donations from 256,000 individuals, with an average donation of $49.

In the third quarter, the campaign said 56 percent of its donors were new and that the average donation was down to $44.

The campaign also said the last week of the third quarter was its best week for online fundraising since early May, something not atypical for campaigns as they press online supporters to make last-minute donations before the end-of-quarter deadline.

The influx also came as Biden became the center of a growing impeachment effort in Congress over President Donald Trump’s apparent effort to pressure the government of Ukraine to investigate his potential Democratic foe.