MANCHESTER, N.H. — Frustrated and flailing as his candidacy threatens to slip away, Gov. Chris Christie of New Jersey is embarking on a scalding effort over the next week to discredit Senator Marco Rubio of Florida, the man he blames for undermining his campaign and whose ascendancy he deeply resents.

And Mr. Christie has a secret ally: Jeb Bush.

Mr. Christie, whose White House hopes hinge on a strong showing in New Hampshire, is unleashing the kind of cutting and personal attacks that brought him renown in New Jersey but that pose a far greater risk in a presidential campaign, where they can harm the assailant as much as the intended target.

Over the past 48 hours, Mr. Christie has mocked Mr. Rubio as a cosseted “boy in the bubble,” derided him as “constantly scripted,” likened him to “the king of England,” and, perhaps most creatively, compared his Senate career to that of a helpless fourth grader who is told which chair to sit in at school.