When ActBlue, the Democratic online fund-raising organization, tried to upload its most recent report to the Federal Election Commission, there were so many contribution records — 955,475 — that the agency’s servers buckled under the load. It took four days and repeated attempts to file the report.

That its monthly filing could tie up the F.E.C.’s servers — a feat previously accomplished only by President Obama’s 2012 campaign — says a lot about the growth of ActBlue. Begun as an experiment by two friends wanting to finance progressive causes, it has become a major fund-raising mechanism for the Democratic Party.

ActBlue has led the movement toward small online political donations. As much as any other organization, it has made such donations easy and common — even, as some donors find, addictive. Because it’s flexible and easy to use, it has also become a platform for little experiments that gently squeeze even more money out of donors.

“ActBlue is very cheap, reliable, portable infrastructure,” said David Karpf, a political science professor at George Washington University who studies online politics. “It fills in very easily and very quickly.”