Donald Trump, the erstwhile Democrat, independent and member of the Reform Party, finally has a fixed partisan identity.

The president may be besieged, unpopular and prone to lashing out self-destructively, but all of this cements his bond to his party rather than erodes it. Commentators who ask wishfully and plaintively, “When will Republicans dump Trump and save themselves?” are missing the point: Trump’s weakness makes him more Republican than ever before.

It was possible to imagine Trump, with a head of steam after his upset victory in November, cowing swamp-dwelling Republicans and wooing infrastructure-loving, anti-trade Democrats into supporting a populist congressional agenda. Maybe this was always a pipe dream given the instantaneous rise of the #resistance against him. But this scenario would have required a strong, focused president marshaling his popularity and driving Congress.

We’ve seen close to the opposite. And, of course, there’s the so-called Russia investigation. “Russia” is a misnomer. The controversy is now shifting from being about supposed Trump-campaign collusion to alleged obstruction of justice and whatever else special counsel Robert Mueller dredges up in what will probably be a free-ranging, yearslong investigation.

So, whatever Trump’s true ideological predilections, there’s no place for him to go. Make deals with the Democrats? At this point, Democrats are more likely to cooperate with Sergey Kislyak on an infrastructure package than with Donald Trump.

Dump or triangulate away from Republicans? Well, then who would do scandal defense, besides a handful of White House aides and outside media loyalists? Imagine what the Comey or Sessions hearings would have looked like if Republicans had joined Democrats in the pile-on.

The need for support on Capitol Hill could well get more urgent if things go badly the next year and a half. If Democrats take the House, Trump will rely on Republicans for an impeachment defense and, if it comes to that, for the votes in the Senate to block removal.

In one sense, this suits Trump. He may have a questionable partisan pedigree, but he is a natural partisan — smash-mouth, heedless of process and norms, willing to make whatever argument suits him at any particular time. There have been many Republicans who have opposed Chuck Schumer before; it took Trump to call him a “clown.”

As for congressional Republicans, they, too, don’t have much choice. Like it or not, whatever they tell reporters privately about their true feelings about Trump, his fate is their fate.

First, a president’s approval rating heavily influences midterm elections. The outcome in the campaign for the House will presumably be much different depending on whether Trump is at 35 or 45 percent. Republicans dumping Trump wouldn’t make him any more popular.

Second, such a distancing is not really politically practicable. If Republicans try to skitter away from Trump, their base will roast them. There’s no reason to think that at this point the dynamic would be any different than after the release of the “Access Hollywood” tape, when Republicans dumping Trump were quickly forced to pick him right back up again.

Third, Republicans want to get some things done legislatively. A poisonous split with the White House wouldn’t help. Trump may be a mercurial and frustrating partner, but he is a partner all the same.

Finally, most Republicans — quite legitimately — think the Russian controversy is a media-driven travesty. If there were a smoking gun, this posture would probably change (obviously, in that circumstance, it should change). But Democrats are in no position to lecture Republicans on cutting loose a president of their own party when they twisted themselves in knots to defend Bill Clinton after he lied under oath over an affair that violated every feminist principle the party professed to hold.

If Trump and Republicans had their druthers, neither would be in quite this position. But this is the reality for everyone. For now, there’s no way out, only through, and through it together.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.