So even though DACA is popular, Republicans would be unlikely to face a backlash among their voters — even their more centrist ones — should they refuse to pass a replacement.

Indeed, Republican members of Congress could face a backlash if they pass one — in the form of primary challenges. In recent elections, a hardline stance on immigration has proved to be a winner in Republican primaries. It has been highly correlated with how well GOP senators have done against primary challenges — senators with more hardline positions have done better against primary challengers; those with more moderate views have done worse.

In 2016, moreover, immigration may have been the issue most responsible for Trump’s winning the Republican nomination. In every state with a caucus or primary exit poll, he did best among voters who said immigration was their top issue.1