The appearance, coming just before Joseph R. Biden Jr. will accept the party’s nomination, has stirred anger among some progressives as well as former Bloomberg campaign workers — and has reignited questions about the multibillionaire’s pledge to throw his fortune behind the general-election effort to defeat Mr. Trump.

“He stiffed our party and all the monthly workers he promised to keep on through November,” Amy Siskind, a prominent progressive activist, wrote on Twitter. “Why is Bloomberg speaking?”…

“After spending a billion dollars on his own candidacy in the primary, many in the party thought that would imply spending at least as much on the general election, if not more,” said Eleanor Neff Powell, an associate professor at the University of Wisconsin-Madison with a focus on money in politics. “A billion dollars may be an unreasonable expectation, but he set — and in some ways expanded — those expectations during the primary, even if he didn’t outright say how much he planned to spend in the general.”