“It was a pivotal issue in the campaign,” said a former Senate aide who supported Rubio back then. “I can’t tell you how excited people were — conservatives on the Hill — to have a conservative Hispanic person running for the Senate who not only stood up for their values, but was willing to do that passionately and eloquently in the most daunting of venues.”

Speaking of Rubio’s new stance on immigration, the aide adds: “I honestly don’t know how to explain it. I’ve never seen anybody so passionately argue against amnesty and then completely flip in two years. It’s just mind-boggling.”

Erick Erickson, whose RedState.com organized “money bombs” to help Rubio in his campaign for the Senate, seconds this view. “I think he has completely reversed himself on the position,” he says. “It’s somewhat bothersome that he refuses to admit a reversal or even an evolution. Somehow trying to reconcile his former opposition to now, it cheapens his image, and I don’t know that he understands that.”

At a closed-door meeting with conservatives two months ago, Blackwell said he warned Rubio that conservatives wouldn’t be able to “betray” their donors and members and support something that didn’t have “actual, foolproof border security in place.”