On April 30, the day Mr. Sanders announced his candidacy, he generated more than $1 million via ActBlue. He got $1.6 million on May 3. Not counting those two largest days, on average ActBlue users sent $177,598 to Mr. Sanders each day between May 4 and June 17.

Without the support of a “super PAC,” Mr. Sanders’s total may seem small compared with some of the other 2016 candidates, but anywhere close to $10 million would be a very respectable total for a candidate who raised a total of $6.2 million for his 2012 Senate race. One question is whether this initial flood of donations represents the high-water mark or can be sustained.

Unlike candidate filings, ActBlue’s F.E.C. reports don’t make for easy browsing, because they often include tens of thousands of rows on a spreadsheet detailing millions of dollars of contributions. And absent special congressional elections in Mississippi, New York and Illinois, ActBlue would not have needed to file its own report until next month. But because of those races, we get a glimpse of donor support for Mr. Sanders.