Scott Walker just exited the Republican presidential race, so who gains? The only other solid conservative with a serious claim to electability: Marco Rubio.

Ben Carson and Carly Fiorina don't have the actual record Walker supporters like. Trump's views on entitlements don't jibe well with Walker's true-believing conservative donors. There's an outside chance Ted Cruz could pick up some of the remnants, but Walker fans also touted his electability, something the junior senator from Texas can't really boast. Only Rubio really fits the bill.

And sure enough, Walker's New Hampshire co-chair has already signed up:

Rubio campaign manager Terry Sullivan just said they've already had an endorsement from Walker's NH state co-chair — Jon Ward (@jonward11) September 21, 2015

And he won't be the last, according to GOP insider and RedState.com proprietor Erick Erickson:

Most of the high dollar Walker donors I've talked to had Rubio as their second choice. — Erick Erickson (@EWErickson) September 21, 2015

Rubio also tweeted condolences of sorts to Walker in a fairly transparent outreach attempt at his supporters. But he was also gaining before this all happened; Rubio is the only establishment contender to have gained in the polls since the last debate, as the rise of Carly Fiorina continues and Trump holds his lead.

It's too early to declare that Rubio has sewn up the Republican establishment. Jeb Bush is still kicking, and Chris Christie has a very low but ultimately nonzero chance of catching on. But at the moment, Rubio is the dark horse — and if insiders unite behind him, he may have a shot at beating Donald Trump.

Rubio is almost creepily perfect, and Democrats are scared of him

On paper, it's kind of bizarre that Marco Rubio isn't already running away with the Republican nomination. He was elected by a wide margin in the biggest swing state in the union. He doesn't have Jeb Bush's last name and associated baggage. He's got experience in elected office, unlike Carson, Trump, or Fiorina. He has not made literally every member of the GOP establishment hate him, the way Cruz has (and Rand Paul has to a lesser extent). He's charismatic on the stump, unlike John Kasich or the dearly departed Scott Walker. He's not insistent on refighting culture wars Republicans have already lost, like Mike Hucakbee is, and unlike Chris Christie, none of his aides are facing federal indictment. And best of all, he's Latino, which helps address Republicans' biggest problem in presidential elections: their supporters are more or less all white, and the electorate is getting less white every day.

That's why Democratic operatives occasionally express outright panic about the prospect of facing Rubio in a general election. In a widely circulated post, Steve Schale — Florida director for Obama '08, senior adviser to Obama '12 — declared, "Marco Rubio scares me. … If you are a Democrat, he should be the one you don’t want to face, because I do think, if he is the nominee, he is the one who could significantly change the Hispanic math in Florida and the Latino math out west. Why? I truly believe he will benefit from the same identity politics that galvanized African American voters behind Obama."

This actually isn't a bad heuristic. If Democrats think Rubio is going to be the toughest candidate to beat, that might be an important data point for Republicans in search of an electable nominee to consider.

Republican primary voters WILL consider electability

Rubio's biggest weakness with the primary electorate is certainly his involvement with the 2013 immigration reform effort. Signing onto a path to citizenship cost him conservative supporters who view that as tantamount to amnesty, while his subsequently waffling left immigration advocates thinking of him as somewhat less than an advocate. But while it's definitely a weakness, previous nominees have overcome worse. Remember Mitt Romney suddenly flipping from pro-choice to pro-life in between 2002 and 2007? Or John McCain's support for gun control?

In general, primary voters are much more pragmatic than they're often given credit for. Political scientists have found, at least since the 1984 Democratic contest in Iowa, that primary voters give electability real weight in picking candidates. And at least as of 2012, that was true of Republican primary voters as well. On December 3-6 2011, political scientists John Sides and Alex Lundry, along with the polling firm YouGov, survey prospective Republican primary voters. Some of them were given Intrade betting market data suggesting that Mitt Romney and Newt Gingrich stood better chances of winning the presidency than Rick Santorum or Ron Paul; others didn't get the info. Support for the more electable candidates jumped among voters shown the Intrade data — with Romney in particular gaining.

Sides notes that "nearly half — half! — of Romney’s supporters actually placed themselves ideologically closer to Newt Gingrich or Rick Santorum. But nearly all of these voters also believed Romney could beat Obama, and that was apparently enough to put them in his camp."

It's hard to evaluate Rubio's comparative electability with current polls, as his name recognition is much lower than Donald Trump's or Jeb Bush's. But there are strong prima facie reasons to think that a charismatic young Latino candidate would garner voters that a 70-year-old xenophobic real estate mogul couldn't. Republican candidates are going to be voting in large part on electability, and to the extent that they do, Rubio is well-positioned to pick up that support.

And it's worth keeping in mind that Rubio, while charismatic and less extreme-looking than some of his rivals, is actually really conservative. He wants to ban abortion and not provide exceptions for rape or incest. He'd cut taxes so much — largely on rich capital gains earners — that the debt would increase $4 trillion over ten years. He is no moderate. That's good news for conservative primary voters, and terrifying to Democrats who think he's especially electable.

Rubio's locking down talented young conservative staffers

If you're a young, 30/40-something Republican in search of a job in the next administration, picking a presidential campaign with which to align can be a tricky endeavor. More than anything else, you want someone you think has a good shot at the nomination and the presidency, and thus a good shot of appointing you to a government post. That makes ambitious would-be aides' endorsements somewhat useful as an indicator of who DC GOP elites think has a shot at winning the general. And the Republican party's fastest-rising young wonks are breaking toward Rubio.

For example, one of his chief foreign policy advisers is Jamie Fly, the former executive director of the Foreign Policy Initiative, which was founded by neoconservative foreign policy insiders Bill Kristol and Robert Kagan. If you want a foreign policy adviser with strong ties to the neocon world, it's hard to do better than Fly. Rubio's also hired Lanhee Chen, who ran policy for the Mitt Romney campaign last cycle and is a shoe-in for domestic policy advisor or some other domestic policy job in the next Republican administration. Oren Cass, Romney's domestic policy advisor, doesn't work on the campaign but designed the Flex Fund and wage subsidy proposals that Rubio has endorsed.

If the feeling was that, say, Jeb Bush was going to unify establishment Republican voters and donors, there'd be little point in ambitious policy types flocking to Rubio. That only makes sense if the feeling is that Rubio is well-positioned to become the consensus establishment candidate, freezing out Bush and Christie.

Again, Rubio doesn't have this locked up. But if you believe in the invisible primary, there are worse horses on which to bet.