New Jersey Gov. Chris Christie had a warning for about 150 Republicans packed into a sports bar here over the weekend: Think before you vote.

“You want to vote for a first-term senator again after watching the last seven years with a ringside seat? Okay. But don’t expect a different result,” Christie said. “The definition of insanity is doing the same thing over and over again and expecting a different result.”

The jab was aimed squarely at Sen. Marco Rubio (R-Fla.), the first-term lawmaker who leads Christie and other “mainstream” Republicans here in Iowa and in New Hampshire. Rubio, Christie, former Florida governor Jeb Bush and Ohio Gov. John Kasich are locked in heated competition to emerge as the alternative to insurgent GOP front-runners Donald Trump and Sen. Ted Cruz of Texas. On the trail and the airwaves, the four-way rivalry has become a twisted web of nasty one-on-one feuds that threatens to do lasting damage to the stability of the ­Republicans’ establishment wing.

“The whole goal is to emerge from the establishment lane,” said Al Cardenas, a Florida-based Bush ally who has been friendly with Rubio for years. “For Rubio, Kasich and Christie, that means getting through Bush. For Bush, it means getting through them. That’s what’s happening. It’s a true four-person race to be one of the top two candidates coming out of New Hampshire, to go into the next few states.”

The Bush-Rubio feud has been simmering since the senator jumped into the race, defying supporters of the former governor who warned him to stay out. Bush’s super PAC is airing sharp attack ads against Rubio, Christie and Kasich that raise doubts about their experience. Kasich is more of a threat to his three rivals in New Hampshire, where he is polling well and picked up three newspaper endorsements Sunday. The Christie-Rubio fracas has intensified since the two clashed onstage at last week’s GOP candidates’ debate.

Campaigning in New Hampshire and Iowa over the weekend, Rubio voiced his sharpest criticism yet of Christie. At event after event, he pulled a piece of paper from his pocket with a 2009 line from the New Jersey governor voicing support for Supreme Court Justice Sonia Sotomayor.

“I have his quote right here. I have it right here,” Rubio said at a Henniker, N.H., town hall, eager to share his finding.

“I support her appointment to the Supreme Court and urge the Senate to keep politics out of the process and confirm her nomination,” he continued.

Meanwhile, outside Rubio events in the same two states, Christie supporters handed out copies of a 2013 fundraising email that Rubio sent on behalf of Christie, arguing that the Florida senator is being hypocritical and nakedly political in hitting him now.

“FLASHBACK: Marco Rubio: ‘Governor Christie has paved the way for limited government reform in New Jersey’ ” was splashed across the handout in bold letters.

Bush is statistically tied with Christie in Iowa and New Hampshire for fourth or fifth place, depending on the survey, and is also jabbing at Rubio. When a voter asked him about the Florida senator during a stop in Ankeny, Iowa, last week, Bush started by saying, “I like Marco. We live almost in the same Zip code and I admire him. He’s a great politician.

“But has he ever had a chance to make a tough decision? I don’t know, think about it,” Bush added. “Has he had to deal with eight hurricanes and tropical storms?”

Another voter in Ankeny asked Bush whether “the responsible candidates, the grown-up candidates” have an obligation to unite behind one person to take on Trump.

“There’s always big swings at the end,” Bush told the man. Later, he urged the crowd to be patient, because the process “will take care of itself” in the coming weeks.

On the stump, Bush is far less aggressive than Christie or Rubio, leaving the harshest attacks to the super PAC supporting his bid, Right to Rise USA. The multi­million dollar operation distributed an orange flyer to people at Bush rallies last week comparing the former governor’s gun-rights record to those of Rubio, Christie and Kasich. A TV ad in heavy rotation across Iowa and New Hampshire portrays Rubio as a flip-flopper on immigration reform.

In response, Rubio portrays Bush as a candidate on his last legs. “We all see what’s happening,” Rubio said in one of his most recent Iowa TV ads. “Jeb Bush is desperate and spending millions on false attacks. Don’t fall for it.”

From his stronger polling position, Rubio is both batting away establishment foes and wooing conservatives in hopes of chipping away at Cruz’s support. At a town hall Friday in Claremont, N.H., Rubio said he was “angry” and “upset” about the direction the country has headed in recent years.

“We are going to make America again the most competitive place in the world,” he said, a line that bore echoes of Trump’s “Make America Great Again” slogan.

Rubio is also making a big push to court Christian conservative voters, who make up a hefty chunk of the Republican electorate in Iowa and have mostly aligned with Cruz.

“If I have to make a choice, I am going to choose life,” Rubio says in a new Iowa TV commercial that emphasizes his opposition to abortion rights.

At his Saturday event at the sports bar in Fort Dodge, the last question Christie got was: Who would you pick as your vice president?

“I think it’s actually nice that I’m being asked,” Christie replied. “Because over the summer no one’s asking me who I’d pick for vice president, ya know? I was at 1, 2 percent in Iowa, and they weren’t going, ‘Who’s your vice president going to be?’ They were like, ‘When are you going away?’ ”

Christie said that if he gets to pick a running mate — still a big if, since he is now at only 4 or 5 percent in Iowa — he wants someone who could do the job and someone he likes. In a final jab at Rubio he added: “I’d like to have another person with executive experience of some kind.”

Robert Costa in Washington contributed to this report.