Priebus is in the occasionally unpleasant position of always having to put a positive spin on his party's chances, so it's not clear if he actually believes this argument. But let's take him at face value. He thinks Trump could win a general election because there is a group of "disengaged" Americans — which is to say, Americans who vote rarely or never — that will show up to give him their votes.
We looked at this a bit last week. At that point, we were skeptical that Trump would be able to turn out voters for the caucuses given that he didn't have access to a voter file. Without a voter file, the campaign couldn't know which voters always turn out (meaning there's no need to get that person to the polls) or which never turn out (meaning they shouldn't be a priority). But that same day, Trump got access to the RNC's file, making that point somewhat less important.
But it is still worth remembering who makes up Trump's base of support in the Republican primary: Disproportionately lower-education and lower-income whites. And the graph below is a reminder that people who earn more in income are more likely to vote.
Data from the Census Bureau shows that less-educated voters also turn out less frequently.
Of course, less-educated and lower-income voters also tend to vote Democratic. In 2012, 64 percent of those who weren't high school graduates backed the Democrat in their House race; 53 percent of those with a high school degree did the same. Majorities of those with some college education or a college degree backed the GOP. (Post-grads backed the Democrats.) That overlaps with race, too, as we noted after the 2014 elections.
We'll note that Trump's advantage among less-educated voters extends to a hypothetical match-up with Hillary Clinton, too, according to Quinnipiac polling from last month.
And finally, we'll note that Trump's support has some crossover appeal to Democrats, according to research reported by the New York Times. It's not enough to swing a head-to-head match-up with Hillary Clinton — match-ups that Trump usually loses — but it exists.
In summary, then: Trump's base of support is among groups that tend to vote less frequently than other demographic groups. The theory is that turning them out means a jump in support for Trump that's hard to measure.
But it's also very hard to turn them out. Less-educated voters likely vote less regularly because they tend to earn less and tend to be younger. That means that they are more likely to rent or more likely to move around or more likely to work odd hours — all of which make it literally harder to vote. (This is why Democrats advocate for early-voting options: Their voters are more likely to need them.) So it's not just a function of getting people excited to vote. The Republican Party would need to actually make sure it's easier for Trump's less-frequent voters to get to the polls, but without increasing the Democratic vote at the same time. That means a good field/turnout program — which is not a Republican strong suit.
The Hill reported Iowa Republicans saying that Trump's ground game "is the real deal," with one Iowa Republican saying that "[t]he question is whether the people they’ve identified as supporters will turn out." And he's "not convinced they will," which undermines the argument that Trump's ground game is the real deal, since a real-deal ground game would be one that would definitely turn out voters.
That is the final card that Priebus gets to drop on us here, though — a card which supersedes all the other cards on the table. Donald Trump almost certainly can't win the nomination unless he demonstrates an ability to get less-frequent voters to the polls in the primaries, including in Iowa. If Trump can do that, he can win the nomination — and Priebus's confidence in general election turnout is warranted. If Trump can't do that, he won't be the nominee and Priebus doesn't have to worry about it anyway.
Priebus's job is to put a good spin on his party's chances. In this case, though, he doesn't need to spin at all.