After a yuuuuge win in Indiana last night, Donald Trump got the validation he has sought all along. Reince Priebus released a statement later in the evening announcing the wire-to-wire frontrunner as “the presumptive GOP nominee,” and called on the party to unite behind Trump:

.@realDonaldTrump will be presumptive @GOP nominee, we all need to unite and focus on defeating @HillaryClinton #NeverClinton

— Reince Priebus (@Reince) May 4, 2016

The unity part will take a while longer (about which Jazz will have more to say shortly). With Ted Cruz getting out of the race last night, that process will start shortly, even if John Kasich wants to keep campaigning in an effort to forestall the inevitable. With Cruz out of the fight, Kasich has no way to hold back Trump, which is why many of us shook our heads at Kasich’s insistence on campaigning against Cruz at all. While the #NeverTrump contingent insists that they will fight to the convention, there’s no way to win. They’re about to become the PUMAs of this cycle — principled opposition with no realistic alternatives in front of them.

Nevertheless, Trump will have to work for party unity in order to win the election. The most significant tool Trump has to move forward on that project will be his choice of running mate. It worked for Ronald Reagan in 1980 when he picked his main rival to join him on the ticket, combining the grassroots conservative movement with the northeastern-GOP establishment. It worked again for Bush when he selected Dan Quayle to keep his connection to the conservative movement, even if Quayle turned out to be less impressive on the campaign trail and especially in a debate. One can even make the argument that Bob Dole tried the same formula with the late Jack Kemp, but that clearly didn’t help much in the three-way 1996 race.

Who will Trump pick, and how will it work to bring the GOP together? The most obvious choice would be to choose Cruz, and that would make some sense at the convention. Cruz controls a lot of delegates, and Trump will need their support for his VP choice to win a nomination vote. After this past week, when the two men fought on bitter personal terms, that seems less likely. Cruz wants to “keep fighting for the Constitution,” and it’s far from clear how he’d do better at that as a powerless VP than in the Senate. If Cruz doesn’t want the job, then the convention won’t be a problem for Trump’s pick, but it could leave conservatives with no reason to rally to the GOP ticket, either.

Failing that, Kasich becomes the next logical choice. Republicans have to win Ohio, and Kasich knows how to do that. After implicitly joining #NeverTrump in his Marvel team-up with Cruz last week, though, that seems less likely to happen. If not, then Trump misses an opportunity to add real executive experience in a political context to the ticket, and perhaps a way to reach past the conservatives and reinforce his strength with party moderates.

My friend Myra Adams offers up Newt Gingrich as a possibility:

Gingrich is the most recognized and respected member of the GOP establishment who is defending the insurgent candidate — even as Trump rails against that same establishment. But now, it has become apparent that Gingrich is waging an active campaign to become Donald Trump’s running mate.

Validating those efforts, Gingrich’s name recently appeared in the pages of the New York Times on a list of “Who Might (or Might Not) Be Donald Trump’s Running Mate if He’s the Nominee.” Which leads one to ask: Could a Trump-Gingrich ticket be a brilliant game-changing winner, or would Gingrich be buying a first-class ticket on the Trump Titanic? …

Gingrich is media savvy and a mega fundraising asset. He is good on the campaign trail and could solidify support among conservatives. Newt and Callista could even help rally Catholics. Most important, Gingrich legitimizes Trump’s candidacy and would refine Trump’s somewhat unartful domestic and foreign-policy positions. And if the current general-election polls are correct, and Trump has alienated Hispanic and female voters to the point where the Republican ticket is going to be soundly beaten — Gingrich could possibly use some of his own political capital to help minimize down-ballot losses by stumping for endangered Republican senators and members of Congress.

Perhaps, but Newt Gingrich was yesterday’s news in 2008. He’s sharp as a tack and a great idea man, but he’s even older than Trump (72 and 70, respectively), who’s older than Hillary Clinton (69). It’s hardly a choice to appeal to younger voters, and might lose more of them. It’s also hard to imagine that Gingrich would be able to “minimize down-ballot losses” from the platform of the ticket that might create them, or that his appeal extends to that level outside of older and/or more conservative districts. All that would be needed would be for Hillary to choose Bernie Sanders as her running mate, and we can have the Battle of the Baby-Boomer Zombies in November. Plus, Gingrich has hardly been seen as a uniter in his decades-long career, no matter how many other accomplishments he has.

If not these, then who can help Trump pull the GOP coalition together? A few thoughts:

Republican governors – They got run off the road by the Trump Express, but there’s still a lot of talent on the GOP bench. Kasich certainly qualifies, as does Chris Christie, but the latter won’t unify anything, and a NY/New Jersey ticket could alienate voters from everywhere else. Scott Walker probably won’t do it, but he’d make a very good choice. Oklahoma’s Mary Fallin or South Carolina’s Nikki Haley might be persuaded. Both are on second terms, and Haley in particular has handled the national spotlight with aplomb. Haley’s been critical of Trump, but she could open doors to younger general-election voters and women that Trump has more or less slammed shut over the last year.

Republicans in Congress – This seems less likely a path for an anti-establishment nominee, but a partnership with Mike Lee or Tim Scott might go a long way to building trust with conservatives. Paul Ryan’s name will come up, but Ryan’s not going to go on the same ride again, and especially not with Trump. Marco Rubio will get a lot of attention here as well, and perhaps might be persuaded to sign onto the unity project. That could force Trump to explain why he’s bringing on the man he accused of being an amnesty supporter onto a hardline-immigration campaign, but Trump’s never had much reason to explain his contradictions in this race before now.

Outsiders – Ben Carson comes to mind, but he proved to be a weak campaigner — and not the best of surrogates, either. Sarah Palin’s name might come up, and Trump might be tempted to think that she’ll pull in more people than she realistically reaches these days … but Trump’s already reaching those voters now. There’s also the possibility that Trump adds to the celebrity quotient by picking a media figure, although that impulse might be tempered by Trump’s lack of willingness to share the spotlight. In that, he’d be no different than any other major-party presidential nominee, either.

Frankly, it’s difficult to determine who Trump will pick, as he has been mercurial and unpredictable for most of this campaign. Let’s see what Hot Air readers think. Who would make the best running mate for Trump? Take the poll, and add your suggestions in the comments below.

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