WILL SEN. MARCO RUBIO (R-Fla.) ever stop atoning for his apostasy in having supported an overhaul of America’s broken immigration system? Or is he so politically pliable and ideologically biddable that he will say anything, and take any stance, to shield himself from the ugly nativism Donald Trump has tapped among Republican primary voters?

It was only in the spring that Mr. Rubio, speaking in Spanish to Univision, deemed “important” President Obama’s program to allow young people who entered the country illegally before age 16 to apply for work permits. He added: “It can’t be terminated from one moment to the next, because there are already people benefiting from it.” 

Now, speaking in English in New Hampshire, Mr. Rubio has revised his views, saying the program should in fact be ended unless broad immigration reform is enacted — precisely the sort of broad reform he advocated in 2013 and later rejected in the face of right-wing outrage.

Let’s be clear: By cancelling the program, known as Deferred Action for Childhood Arrivals, Mr. Rubio would also expose to deportation proceedings the roughly 700,000 youths who have benefited from it. That means 700,000 young people, often known as Dreamers, who have grown up in this country, attended school and in many cases college in the United States, and who know no other country. 

Mr. Rubio’s craven flip-flop is in keeping with other rightward tilts in the GOP primary field, particularly on immigration, as the candidates scramble not to be outflanked by Mr. Trump. Even former Florida governor Jeb Bush, while not repudiating his support for comprehensive immigration reform, now downplays talk of providing a path to legal status for undocumented immigrants, instead emphasizing almost exclusively beefing up border and interior security.

Mr. Rubio’s retreat has been especially stark given the prominent role he played as a member of the Gang of Eight bipartisan senators who wrote the 2013 immigration reform legislation. That bold bill, which passed the Senate but went nowhere in the House, elevated his status among establishment Republicans and many independents; it also put him in foul odor with the party’s anti-immigrant wing now so enamored of Mr. Trump’s hate-filled rhetoric.

As the GOP presidential nominee in 2012, Mitt Romney said he would not deport young people covered by Deferred Action for Childhood Arrivals (although, notoriously, he urged other illegal immigrants to “self-deport”). Yet under Mr. Trump’s malign influence, similarly moderate stances on immigration are now regarded as political self-destruction.