Some of Rubio’s closest allies privately fear the Florida Republican waited too long — and that his newfound focus on Trump is the only way to revive a campaign that is on the ropes.

“Should they have gone after him? Yeah. Would it have been effective? I don’t know,” said one of Rubio’s Senate colleagues. “We don’t know this is going to be effective. Maybe there isn’t a magic tea to tell voters that Donald Trump is a huckster.”

Senior Rubio campaign officials and people close to the campaign said Rubio had little choice but to wait until after the field narrowed to go after Trump. They said they were taking sustained fire from the likes of Jeb Bush, Chris Christie and Ted Cruz, and were forced to respond in kind.

Moreover, they argued, had Rubio focused on Trump with a more crowded field, it only would have served to benefit Trump, who was dominating the media coverage, and would have done little to improve Rubio’s own standing.