New Jersey governor Christie endorses Trump

New Jersey governor and former Republican candidate Chris Christie has announced he will endorse frontrunner Donald Trump for president.

"I'm happy to be on the Trump team and I look forward to working with him," Christie told a Trump rally in Texas, adding that Ted Cruz and Marco Rubio, both running for president, were "unprepared" for the top job.

“There is question that Mr Trump will turn around Washington and keep [Democratic candidate and former Secretary of State] Hillary Clinton from winning the White House,” he said.

Christie dropped out of the 2016 presidential race after a lacklustre showing in polls and state races.

"He is rewriting the playbook of American politics because he's providing strong leadership that is not dependent upon the status quo,” he said on Friday.

"I will lend my support between now and November in every way that I can for Donald, to help to make this campaign an even better campaign than it's already been."

"Generally speaking, I'm not big on endorsements, but this was an endorsement that really meant a lot,” the New York businessman said of Christie.

Donald Trump is leading in many state polls and has already won three consecutive state contests in New Hampshire, Nevada and South Carolina, despite never having worked in politics.

He shouldered heavy criticism from Rubio and Cruz at a Republican debate on Thursday but it is not yet clear whether this has hurt his popularity.

Florida Senator Rubio continued to assail Trump the morning after the debate, telling CBS that “a con artist is about to take over the Republican Party and the conservative movement, and we have to put a stop to it."

At a morning rally in Dallas, he mocked Trump about his misspelled tweets and said he had seen him having a "meltdown" backstage at the debate.