“If DeMint wins, Rubio loses,” said Frank Sharry, the executive director of America’s Voice, a pro-immigration group. “The Republicans’ best chance going forward is to have Marco Rubio win this day, and if Jim DeMint cuts him off at the knees, he’s going to hurt Republican chances of regaining the White House.”

Others see their relationship as that of a young, charismatic son (Mr. Rubio) eclipsing his more experienced father (Mr. DeMint).

“It’s almost like a Greek story, where the offspring is now leading the very thing his father made his bones over,” said a Senate Republican aide, speaking anonymously in order to offer a candid assessment. “The leader who killed the 2007 bill — that was his one accomplishment. And now his protégé is picking up the baton in the exact opposite direction.”…

Though loath to take on Mr. DeMint himself, Mr. Rubio and his staff are already pushing back on his economic argument, saying that the public benefits of an immigration overhaul would outweigh any concerns about cost. Mr. Rubio said that most immigrants, “once they become legalized and improve their lives, they’ll be better. In essence, they’ll pay more in taxes because they’ll be making more.”