Senator Marco Rubio of Florida, who on Thursday found himself under a double-barreled assault from Jeb Bush and Gov. Chris Christie of New Jersey over his preparedness to be president, chalked up his rivals’ attacks to fear that they will lose to him in the New Hampshire primary next week.

And he pointed to his experience on the Senate Intelligence Committee — and the access he has to classified information — as a credential that no one else in the race has.

“I think it’s silly. That’s how I respond,” Mr. Rubio told reporters about Mr. Bush’s and Mr. Christie’s criticism. “Look, when people are having a tough time in a campaign, especially near the end, you see some desperation set in and so they’ll start saying things. But I’m not too worried about that.”