Serious presidential candidates don’t quit running because they lose a few primaries. They quit because they run out of money.

Most large donors are strategic. There’s zero value and even a stigma attached to being a big donor to a losing campaign. When candidates have a bad news cycle or underperform in a primary, large donors zip up their pockets. They wait and see if the candidate will recover. Those are harrowing times for campaigns, which must cut staff and advertising, give up the jet, and try to hang on. After poor showings in Iowa and New Hampshire, it must be hell for Joe Biden’s fundraising team.

Mike Bloomberg is uniquely immune to donor withdrawal. The former mayor of New York has committed to reach into his $54 billion pocket and spend whatever it takes to win.

Mr. Bloomberg has flipped the donor’s traditional money-for-access-and-influence proposition on its head. Wealthy donors are invited to join his campaign as supporters and advisers—without writing checks. Instead the proposition seems to be access and maybe influence in exchange for not contributing to other campaigns. If Mr. Bloomberg has a good shot to win the nomination, that’s a cost-effective arrangement for donors. They don’t have to risk giving money to a losing campaign.

But this trade overlooks a secondary function of campaign contributors. Donating money is a tangible commitment. When invested, donors of all kinds are likelier to tell others, work for the campaign and vote. There aren’t enough big donors for these effects to move the needle. But with small donors, these outputs are essential ingredients in the movement that generates the vote.