Let’s presume this deal only became possible after Marco Rubio dropped out—after all, the Florida senator’s last-ditch efforts to direct support toward Kasich were rebuffed. The next major contest was Arizona, a winner-take-all state that Trump won with 46 percent of the vote. Kasich and Cruz, as much as they tried, only got a combined 40 percent. No amount of vote-swapping would have changed that outcome.

New York, the next stop in our time machine, looks a bit more promising. Kasich won parts of Manhattan and Queens in New York City, albeit by a mere 70 votes, and came close in several counties upstate. Some extra juice from Cruz could have pushed the Ohio governor over the top in a few districts, though probably not enough to top Trump’s 60-percent lead statewide.

Paradoxically, Kasich could have actually performed worse in an Empire State primary without Cruz in the competition. The polls don’t uniformly agree, but at least one pre-election survey showed the average New York Cruz supporter would pick Trump, not Kasich, if forced to drop the Texan. While Kasich performed better against the New York billionaire than Cruz statewide, Cruz’s people preferred Trump as their second choice in this early survey. (Exit polls don’t ask similarly clear questions about voters’ second-choice candidates.) That’s a warning worth heeding in later contests, where Trump support could pop up where it’s least expected.

That takes us to Indiana, where Kasich will bow out and let Cruz take the lead. Cruz does poll better there than Kasich does; the Ohio governor’s 16 percentage points of support could prove pivotal in closing the gap between Trump, who is currently polling at 41 percent, and Cruz, who has 33 percent.

But there’s no guarantee this will pan out as planned. Remember: People don’t like Ted Cruz. Only half of Kasich supporters say they’d vote for the Texas senator. Only a third of voters say they’d even be willing to vote for someone other than their first choice, period. In the best case scenario, the Texas senator could win only a few extra percentage points of support, well below what he would need to beat Trump.

There’s not much polling for New Mexico and Oregon, but Nate Cohn at The New York Times notes the Kasich-Cruz bargain could bolster Trump, as it could have in New York. Not that it makes much of a difference, given those states’ delegate counts are teeny-tiny and proportionally allocated.

Mr. Cruz has fared very well out West, and he might have been favored to win either or both states over Mr. Kasich and Mr. Trump. Mr. Cruz’s concession would seem to increase the chance that these states go to Mr. Trump. But this has virtually no downside to anti-Trump forces, at least from a delegate perspective. That’s because New Mexico and Oregon are the only two states after Rhode Island on Tuesday that award their delegates on a purely proportional basis — meaning they award their delegates in proportion to a candidate’s statewide share of the vote.

But this is all exceptionally squishy to forecast. After all, who is to say that Kasich and Cruz voters will play along with this bargain? It’s a lot to ask an electorate to pick you for president. These two men are going even further, essentially telling voters to cast ballots for another guy, so a third guy won’t win, so they’ll have a shot at winning later, maybe.

You can’t be half pregnant, the saying goes. If you’re a political candidate, you also can’t be half dropped-out. The Cruz-Kasich alliance would be more compelling if one of them actually left the race and threw their weight behind the other. As it stands, their voters are stuck in a marriage of convenience that neither man really wants.

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