Jeb Bush may feel he has no choice. Donors have invested tens of millions of dollars in his campaign, and many of them must, by now, be getting uneasy. If the uneasiness intensifies, the flow of checks will slow. Every campaign runs on money, but Jeb Bush’s more than most. He’s hoping through sheer staying power to impose his nomination on a recalcitrant party. The money buys the staying power. If the money dwindles, his fortunes fall hostage to the Republicans of the early states: Iowa, New Hampshire, South Carolina, and Nevada, none of them a promising Jeb Bush firewall.

So the attack-Trump mistake may be unavoidable, a necessary error to avert the even-worse calamity of having his fundraisers go on strike. Yet attack-Trump remains an error all the same for Jeb Bush—and for two big reasons.

First, Trump certainly hurts Bush, but he hurts other candidates more. Bush’s most immediate problem is not that the base doesn’t trust him—it didn’t trust John McCain either, yet he nevertheless won the nomination—but that his donors enjoy too many plausible alternative choices. Bush needs to hustle the donor-acceptable alternatives out of the race as fast as possible, as Mitt Romney was able to do in 2012, leaving donors with a stark alternative: me or some sure-loser madman. Once the donors are locked up and locked down, the party base will sooner or later have to submit. In 2012, as Bachman, Herman Cain, and Newt Gingrich exploded on the tarmac, base voters swung to support Romney.

Bush’s trouble is that he doesn’t have the clout to push the other donor-acceptable alternatives out of the race. Real-world politicians quit when they see they can’t win, and except for Rick Perry and George Pataki, none of the donor-acceptable alternatives will look at today’s polls and think, “I can’t win.” So Carly Fiorina and John Kasich and Marco Rubio and Scott Walker will linger in the race, subdividing the big money and waiting to pounce on any Jeb Bush misstep or defeat. Bobby Jindal will hang on too—what has he got to lose?

But if Bush can’t drive out the donor-acceptable alternatives, Trump can. Trump is doing just that to Scott Walker right now. Four months ago, the clever thing to say about Walker was that he was the one candidate who could win both the base and the big donors. That unique strength has proved instead a unique vulnerability, as Walker has been whipsawed by the internal party argument over immigration. Walker—a strong-willed politician, but not a nimble one—has tangled himself in a sequence of contradictory answers. He has tumbled from first place in Iowa to third. Dependent on a smaller donor group than Bush, Walker is also more susceptible to donor panic. Walker is a real-world politician, with a job back in Wisconsin and other life options than the presidency. The candidate Bush supposedly feared most may end as one of the first to exit the race. (Necessary disclosure here: I worked in the White House of George W. Bush; my wife donated—modestly—to Scott Walker’s SuperPAC.)