None of this has reassured the conservative pundits and voters already inclined to mistrust Rubio. Radio show host Laura Ingraham rips him regularly, and Ann Coulter’s new book, Adios America, tears into Rubio for his support of the Gang of Eight plan. Whether or not such pundits speak for the GOP primary electorate as a whole, it’s inconceivable that Rubio’s foes won’t hit him on this issue early and often.

But Rubio could also take fire on immigration from a rather less-expected source: His most natural rival for voters and donors, Jeb Bush, whose moderate immigration views are a perceived weakness of his own.

On a conference call with Alabama supporters late last month, Bush discussed his support for “a path to earned legal status, not citizenship” and accused unnamed figures of “abandon[ing] their views,” a comment that some saw, justifiably, as a shot at Rubio.

If Bush’s comment was indeed aimed at Rubio, then the Florida governor will attempt to argue his record of steadfast support for a path to earned legal status is superior to Rubio’s shifts, and Rubio will continue to find himself squeezed on both sides: from the right by challengers like Scott Walker and Ted Cruz, and from the left by Bush.