WASHINGTON - Joe Biden took in more than $15 million for his White House run over the past two months, a sum that shows the former vice president's fundraising operation has rebounded slightly after a lackluster summer in which he trailed his leading rivals.

Biden's campaign would not say exactly how much he has raised since the end of September. But with roughly one month left before the next reporting deadline, campaign manager Greg Schultz said in a memo provided to The Associated Press that Biden has already surpassed the $15.6 million he raised across July, August and September.

The improvement comes at a costly juncture in the Democratic primary, as candidates sprint to get their message out and mobilize supporters ahead of the first-in-the-nation Iowa caucuses in February.

Schultz said online fundraising helped fuel the increase, which he attributed to discredited attacks that President Donald Trump has made against Biden and his son over their past dealings in Ukraine.

Trump has sought to implicate Biden and his son Hunter in the kind of corruption that has long plagued Ukraine. Hunter Biden served on the board of a Ukrainian gas company at the same time his father was leading U.S. diplomatic dealings with Kyiv under President Barack Obama. Although the timing raised concerns among anti-corruption advocates, there has been no evidence of wrongdoing by either the former vice president or his son.

"His campaign and special interest groups have spent millions of dollars airing ads to spread those same lies," Schultz wrote. "This groundswell of support shows us — and Trump, and reporters, and anyone else watching — that his whole scheme is backfiring."

Democratic presidential candidate former Vice President Joe Biden arrives at a stop on his bus tour, Dec. 2, 2019, in Emmetsburg, Iowa.

The fundraising uptick, however, doesn't offer a complete picture. A more telling sum at this stage of the race would be how much cash Biden has on hand.

The $9 million he reported in the bank at the end of September trailed Bernie Sanders, Elizabeth Warren and Pete Buttigieg — all of whom reported having at least twice that amount.

Yet for anxious Biden allies it shows renewed signs of vigor at a time when many have fretted about whether his campaign could hold on until Southern states, where Biden enjoys more support, hold their contests later in the year.

Schultz said the revenue bump will give the campaign "the resources we need to knock more doors, make more calls, and build more support in the caucus rooms and in the ballot box."

"The resources are important, but the timing really couldn't be better," he wrote.