Noah Feldman is a Bloomberg Opinion columnist and host of the podcast “Deep Background.” He is a professor of law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter. His books include “The Three Lives of James Madison: Genius, Partisan, President.” Read more opinion SHARE THIS ARTICLE Share Tweet Post Email

Photographer: Charlie Leight/Getty Images Photographer: Charlie Leight/Getty Images

Donald Trump's anti-immigrant views have done a good deal for his presidential polling numbers. There's also reason think he's done the country a favor, by forcing politicians on the right to repudiate that anti-immigrant sentiment.

In Europe when such rhetoric rises, it's bad news. Anti-immigrant politics are usually there to stay, and the xenophobia pushes the whole political scene rightward. Denmark is a textbook case. But in the U.S., the opposite seems to be happening.

Sure, the anti-immigrant sentiment is playing well in polls for now. But instead of following Trump, or copying him, the right-leaning candidates with more political experience are staking out pro-immigrant territory or at least anti-anti-immigrant positions.

All this is great. Trump is helping to save the U.S. from the European scourge of right-wing anti-immigrant politics. Which leaves the question: Why is it happening? Political scientists think that politicians are like other market actors, and when they see demand, they step up with supply. Trump's popularity is evidence that there's some demand in U.S. politics for anti-immigrant views. So why, exactly, aren't other right-wing candidates acting to supply this demand?

It would be great to answer that our politicians are deeply uncynical people, committed to the American dream of immigration and free of bias. Or that the U.S. is somehow free of European-style dislike of immigrants. But that would be both naïve and historically obtuse. The U.S. has a history of nativism going back to the 19th century. Paired with isolationism, it makes a plausible if wrongheaded political platform. The Trump polls show that some Americans are just as prepared to scapegoat immigrants as are some Europeans. If another candidate adopted the approach today, instead of Trump, it might well have legs.

But how far would those legs carry that candidate? Here's where the specifics of U.S. presidential elections come into the story. In the strange system we have, the way to win primaries is to appeal to the party faithful, who tend (at least these days) to be more partisan and more extreme than the average voter. Jeb Bush will have to tack right to meet the challenges from Ted Cruz, Rick Perry, Rick Santorum, Bobby Jindal, et al.

Likewise, Hillary Clinton will have to go left in the primaries. When Elizabeth Warren decided not to run, Clinton could've been forgiven for thinking she wouldn't have to play this game -- but Bernie Sanders appeared to pressure her leftward.

But catering to the party's extremes is just the first part of the electoral two-step. In the next segment of the dance, the candidates have to go back to the center to win over the median voter. That might be less true today than it was when, say, Bill Clinton ran for president -- there's evidence that polarization is greater than it was, and fewer voters in the middle are truly undecided. But it remains the case that to win the general election, the parties’ nominees have to bring in more moderate voters than they do in the primaries.

That brings us back to why the conservative Republicans are in the race at all. Some no doubt harbor fantasies of becoming the nominee. But for most of them, the goal is to burnish their images, raise their national profiles -- and compete for the vice-presidential slot.

If Bush is the Republican nominee, he'll come under intense pressure to nominate someone perceived as to his right in order attract conservative voters in the general election. But that person can’t be associated with views so extreme that they would taint Bush in the eyes of more moderate voters. In particular, the vice-presidential nominee can't be anti-immigrant if Bush is to take advantage of Latino goodwill that might be generated by his fluent Spanish, his Mexican-born wife and his half-Latino children.

That's why, even if it looks tempting, no conservative candidate who wants to be vice president can speak the way Trump is speaking. The pull of the center is simply too strong.

In a perfect world, most Republican candidates would prefer to remain silent about immigration, to benefit from anti-immigrant sentiment without saying anything disqualifying. That's why they're angry at Trump -- because he's forcing them to openly reject anti-immigrant politics, thereby losing the support of anti-immigrant voters.

All of this is, of course, a product of the two-party system and the direct presidential election. In a parliamentary system, you can become a big and important party without appealing to moderates at all. Thus the Danish People's Party went from winning just 7.4 percent of the vote in 1998 to 21.1 percent this year -- and in the interim, it participated in a center-right coalition government. It's now an important player in Danish politics despite being perceived as far-right and anti-immigrant by many observers inside and outside the country.

Right-wing Republicans remain unhappy about their party's inability to nominate a true hardcore conservative for president. For now, they'll have to be satisfied with trying to move the party as a whole to the right. And cursing Donald Trump.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

To contact the author on this story:

Noah Feldman at nfeldman7@bloomberg.net

To contact the editor on this story:

Stacey Shick at sshick@bloomberg.net