Second, those voters know from long experience that whatever leading Republican politicians say about immigration and border security, many of them have similar views to, well, Barack Obama: They favor more immigration and less enforcement, and they privately (or not-so-privately) regard anyone who disagrees as a knuckle-dragging nativist.

This includes, again, the last Republican president — on whose watch, as not a few voters still remember, the most spectacular terrorist attack in American history was carried out by a group of men who were here entirely legally, most of them on visas from a country that’s officially one of our closest allies in the Muslim world.

So then you combine those two issues in a presidential campaign, and your party’s leadership (the new speaker of the House, Paul Ryan, very much included) consists of figures who basically still believe in Bush’s approach to both national security and immigration, and the presidential candidates with the clearest support from the Republican establishment are either Marco Rubio (who sounds Bush-esque on foreign policy and tried to pass a comprehensive immigration bill with Chuck Schumer) or Jeb Bush (I repeat, Jeb Bush) … well, you can understand why at least some Republicans (the less ideologically committed, more disaffected sort, especially) feel like they’re being offered something they’ve seen tried already, and why the party’s promise to improve on Obama’s record seems less authoritative and more like another bill of goods.

Which is why, to borrow from a column I wrote a few weeks ago, it’s been hard for the G.O.P. to find someone capable of playing Richard Nixon to Trump’s George Wallace. In theory, the Nixonian move — promising to address the legitimate fears Trumpism is exploiting, while disavowing the bigotry and crankery and proto-fascism — is both the right one and the one that the party badly needs.

But in the party as it currently exists, which prominent candidate has the credibility to play that part? When he made his law-and-order, peace-with-honor pitch in 1968, Nixon could rely on his association with the relative tranquillity of the Eisenhower era. But which prominent Republican can say to anxious voters now, with the ring of authority, you can trust me not to just be an open-borders guy on immigration, and my anti-Islamic State strategy won’t just end up in the same ditch as our last ground war in the Middle East?

That’s close to Ted Cruz’s message, and it does seem to be helping him rise. But Cruz isn’t an experienced statesman, he isn’t an authority figure, he’s running his own kind of populist, “bomb ’em back to the Stone Age” campaign and riding Trump’s wake as he goes. So if he’s like Nixon (and he certainly has some of the awkwardness), it’s the Nixon of the late ’40s and early ’50s, the “pink right down to her underwear” Nixon, which is a rather different sort of thing.