ActBlue, the central platform for donations to Democratic candidates and causes, announced that from July 1 to Sept. 30, it had processed $1.5 billion in contributions — an amount roughly equal to what the site raised during the entire 2018 election cycle, and one far exceeding the $623.5 million that the equivalent Republican platform, WinRed, took in during the quarter.

Mark Kelly, the Democratic Senate candidate in Arizona, was among those who reported raising another enormous sum. Mr. Kelly’s campaign took in more than $38.7 million in those three months, and polls in the state show him with a widening advantage over the Republican incumbent, Senator Martha McSally. His campaign indicated that it had entered October with $18.8 million in cash on hand.

Ms. McSally’s campaign reported raising $22.6 million in the period, with nearly $12.2 million in the bank.