For most of Rubio’s competitors, finishing ahead of him rather than Trump is a more realistic goal in the early states. It’s particularly critical for those stuck in the middle tier to receive a boost with donors and voters in the first wave of contests.

Christie, New Jersey’s governor, is a good example. Since the last debate in December, the Rubio-Christie rift has deepened.

Christie has said in recent interviews with The Washington Post and other news outlets that Rubio “turns tail and runs” (a reference to him walking away from comprehensive immigration reform); that the “truant officer is out looking for him” (a reference to his many Senate absences and perhaps his relative youth); and that Democratic candidate Hillary Clinton would “cut his heart out.”…

“I’m sure he doesn’t really want to have a conversation about the issues,” Rubio said of Christie in Nashua, N.H., last week. Rubio dinged the New Jersey governor as a backer of Common Core education standards, said he “ran for office as a supporter of gun control” and noted that Christie once made a contribution to Planned Parenthood. Days earlier, a pro-Rubio super PAC unveiled a TV ad campaign in New Hampshire with similar lines of attack.