Sen. Bernie Sanders' campaign said Tuesday that the Democratic presidential contender raised $25.3 million during the third quarter, an increase of more than $7 million from his second-quarter haul.

According to the campaign, Sanders' third-quarter total came from more than 1.4 million individual donations. The campaign attributes to boost over its second-quarter total to more than $10 million raised in September alone, a total they said was a record for a single month. The average donation from those donors was $18.07, according to the campaign.

The Vermont independent senator, who sought the Democratic Party's 2016 presidential nomination, has raised about $61.5 million since launching his campaign in February.

"Bernie is proud to be the only candidate running to defeat Donald Trump who is 100 percent funded by grassroots donations — both in the primary and in the general," Sanders' campaign manager Faiz Shakir said in a release. "Media elites and professional pundits have tried repeatedly to dismiss this campaign, and yet working-class Americans keep saying loudly and clearly that they want a political revolution."

The big third-quarter haul comes as Sanders has slipped behind both former Vice President Joe Biden and Sen. Elizabeth Warren, D-Mass., in the RealClearPolitics aggregate of several polls.

Meanwhile, South Bend, Indiana, Mayor Pete Buttigieg's campaign announced Tuesday that the Democratic contender raised more than $19.1 million in the third quarter, a dip from the $24.8 million the campaign pulled in in the previous fundraising period. Buttigieg has now raised more than $51 million since the start of 2019.

The Indiana mayor finds himself in fourth place, behind Biden, Warren and Sanders, in the RealClearPolitics average.