Former Florida Gov. Jeb Bush, who suspended his presidential bid in February after several primary losses, endorsed Wednesday morning Texas Sen. Ted Cruz Wednesday, a man he had criticized frequently while still in the running. Cruz isn’t the only remaining candidate Bush could have endorsed, of course, but what is striking is that Bush seems much closer aligned with the politics of the alternative hopeful, Ohio Gov. John Kasich, and there hasn’t exactly been love between the Bush family and the Texas senator.

From their general demeanor to their stances on immigration and Common Core education standards, Bush and Kasich have sounded pretty similar. As for Cruz, Bush’s brother, former President George W. Bush — a figure who played prominently, if not well, in Bush’s 2016 campaign — has been very critical, telling donors last year he just doesn’t “like that guy.”

So why is Bush backing Cruz and not Kasich?

The most likely answer, aside from the potential that he truly does believe, as he wrote Wednesday, that Cruz is “a consistent, principled conservative who has demonstrated the ability to appeal to voters,” is that Bush thinks the Texan is the only candidate with a prayer of beating front-runner Donald Trump. Given the delegate math, Kasich's path to overtaking Trump is steep.

Following Tuesday’s contests in Arizona and Utah, Trump leads with 739 delegates followed by Cruz, with 465; Kasich has only 143, according to the Associated Press. A candidate needs 1,237 delegates to win the nomination outright at the convention. With his current standing, Trump appears to be the only candidate capable of winning that number. Cruz, polls show, has the best chance of beating Trump one-on-one and keep Trump from hitting that threshold.

Bush’s endorsement is the latest sign that the so-called Republican establishment is coalescing around Cruz’s campaign. If he can keep Trump from getting 1,237 delegates (and doesn't himself reach that marker), the two candidates will head into a contested convention where delegates will be freed up to vote with their conscience.