Hardline Is Winner

Nate Silver's FiveThirtyEight site writes that attitudes toward illegal immigration are complicated. However, they make one very clear statement:

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.

Inasmuch as winning reelection is the prime directive of politics, a hardline stance on immigration is the obvious choice.