A perfect complement to yesterday’s post showing Ryan’s favorable rating now underwater among … Republicans.

It’s Trump’s party now. And by “now,” I mean “for the next 19 days.”

I’ll give you three theories for that, none of them mutually exclusive. One: The GOP’s base isn’t nearly as “conservatarian” as movement conservatives would like. Ted Cruz learned that the hard way in the primaries, right? Much of the base is populist first and foremost, and that populism trends strongly towards nationalist/reactionary politics, not classical liberalism. Republican voters, especially Trump’s white working-class fans, care little for conservative economics as practiced by Randians like Ryan. They’re Republican chiefly because that party is their best vehicle for white identity politics and culture war waged against left-wing political correctness.

Two: It’s immigration, stupid. As America’s demographics have continued to change, the right has become more sensitive to that change accelerating by importing millions of workers from Mexico, Asia, and so on. Some significant chunk of the base has effectively decided that immigration is so urgently important that the right position on borders can excuse the wrong position on virtually anything else. (Ahem.) And Paul Ryan most definitely has the wrong position on borders. His House caucus has held the line against Obama’s executive amnesties, knowing that the revolt among the base would have been even worse this year if they’d caved, but Ryan has been soft on amnesty for years, even partnering with open-borders shill Luis Gutierrez on proposals. If Ryan had been a Sessions-style border hawk all along, he wouldn’t be in this much trouble.

Three: This isn’t about policy, it’s about rank tribalism. A presidential election is political war and Trump, not Ryan, is the general right now. It doesn’t matter which policies Trump and Ryan do and don’t agree on. The hard fact of the matter for many Republicans is that Ryan fragged Trump the day after the “Access Hollywood” tape came out; asking them now who better represents their politics is like asking them if they prefer their commanding officer to the insubordinate who tried to murder him. However, once Trump’s no longer in command, that preference might shift. In fact…

“What is clear in these data is that a large segment of Trump supporters are all-in with the candidate. They see him as capable of delivering on the promise of a greater nation. That said, just 38 percent of them say they will stay loyal and follow his future endeavors if he does not win,” said pollster J. Ann Selzer, who oversaw the survey ahead of the final debate Wednesday. “If he were to lose, our data suggest his standing would diminish.”

Thirty-eight percent ain’t peanuts but Trump’s support within the party is soft enough that he finished second, not first, in Bloomberg’s poll when Republicans were asked who the leader of the party will be if Trump goes on to lose the election. Mike Pence finished on top with 27 percent. Trump was second at 24 percent, then Ted Cruz at 19, then Ryan at 15. (When given a binary choice between Trump and Ryan in yesterday’s YouGov poll, slightly more Republicans said Ryan is the leader of the party, not Trump — another sign of Trump’s support softening.) Trump’s favorable rating within his own party stands at 76 percent, fully 15 points lower than Mitt Romney’s rating among Republicans at this time four years ago. There are some Trump loyalists who will stick with him after the election no matter what — there had better be, for the sake of Trump TV — but there’s also room for Ryan, Cruz, etc to rehab their own images as the tribal pressures of the election ease. In particular, Ryan will emerge by pure circumstance as Hillary Clinton’s biggest headache in the Republican Party. If the Senate flips to the Democrats, as is likely to happen, and the GOP holds on in the House, the only thing stopping Clinton from ramming through her agenda will be Ryan and his caucus. Unless he’s foolish enough to sell out the base on immigration, he’ll have many opportunities to earn back goodwill among Republicans by thwarting their least favorite Democrat. His numbers should rise. But as I say, all bets are off if he caves on amnesty.