But the Democrats are also deeply indebted to one man: Mr. Trump, whose provocations and tirades — along with a loud crusade against his own party’s donors — have virtually shut off what once promised to be a half-billion-dollar spigot of outside money.

“Everyone thought that we would be outspent, that there would be significant operations built at the presidential level for the other candidate,” said Guy Cecil, a former Clinton aide who heads Priorities USA Action, the main hub of big Democratic giving. “That obviously hasn’t happened.”

The biggest groups set up or expanded by conservatives since Citizens United — including American Crossroads, founded by Karl Rove, and the network overseen by Charles G. and David H. Koch — are absent from the presidential campaign, focusing instead on protecting Republicans in Congress.

“Unlike President Obama four years ago, Clinton embraced the outside money game and shrewdly empowered a single group to carry her message,” said Steven J. Law, the president of American Crossroads and its sister organizations, “in contrast to the boardwalk arcade of groups supporting Trump.”