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.)

Thank you, Donald.

The second reason Bush shouldn’t fight Trump: Even if Bush wins, he’ll lose. Jeb Bush is a candidate with many points of vulnerability: personal, familial, financial. Most of Jeb Bush’s Republican rivals will be reluctant to broach these issues in any but the most elliptical way. The norms of American media will inhibit journalists from reporting on them. If Bush can win the nomination, he can rely on the threat of mutually assured destruction to deter the equally vulnerable Hillary Clinton from pressing very hard.

But Trump is not playing by the usual rules. Show Trump a line, and he’ll cross it. He has crossed it. And Jeb Bush is a candidate who needs lines respected almost more than any other.