Chris Christie's endorsement of Donald Trump is a surprise.

The New Jersey governor may have known the Republican frontrunner for years, but until just two weeks ago he was suggesting he wasn't fit to be president.

Now as he stands ready to throw himself into the campaign, he'll be forced to defend some of the things he's been saying about the billionaire businessman.

Trump's main platform is that he is going to build a wall with Mexico to keep out illegal immigrants.

It's become such a theme at campaign events, he now shouts: "And who is going to pay?".

The crowd dutifully respond "Mexico".

But Christie said such a plan made no sense.

He dismissed the idea of the Mexicans footing the bill and told Trump: "This is not negotiation of a real estate deal. This is international diplomacy".

Ridiculous position

Then there is Trump's idea to temporarily ban all Muslims from entering the US.

This gets big cheers at Trump rallies.

Christie's reaction to that? "[This is] the kind of thing people say when they have no experience and don’t know what they're talking about. It's a ridiculous position."

When Trump claimed he'd seen thousands of people in New Jersey - Christie's home state - celebrate the attacks in New York on September 11, 2001, the governor dismissed it simply: "That didn't happen."

Christie also called Trump thin-skinned. And just a few weeks ago in New Hampshire, he dismissed Trump as not fit for the country's top office describing him as a "carnival barker", adding "we are not electing an entertainer in chief".

Trump was so upset with some of the things that were being said, he described Christie as a "former friend".

So why are they now best buddies again?

Well, first of all, Christie is smart enough to read the political lie of the land.

Trump is almost certain to land the Republican nomination and it's better to get on board now than later.

Given his term as New Jersey governor ends in 2017 and he can't stand again, Christie may be keen on a top job in any Trump administration.

Conventional wisdom

Conventional wisdom suggests Christie might even land the spot as vice president.

It is the traditional role for the political attack dog, and Christie is good at that. But Trump will want diversity on the ticket, and so that probably rules out another angry white man in such a key role.

But as a former federal prosecutor, Attorney General Christie might sound good to him.

Christie also severely dislikes the two other main challengers.

He dismantled Florida Senator Marco Rubio on the stage during the New Hampshire debate.

He regards him as a political lightweight, who looks good and says all the consultant-approved lines, but lacks real depth. And Ted Cruz is seen as one of the reasons people hate Washington in general and politicians in particular.

Christie believes he's difficult and obstructionist and puts himself above getting things done. And this is also a real dig at the Republican establishment.

Romney experience

Four years ago when the party was not in love with Mitt Romney as a candidate, Christie was sounded out.

He met big money backers and influential figures who tried to persuade him to run. But whether it was because he'd pledged support to Romney or some other reason, he ducked the fight.

This time around, those moneymen and big-name supporters didn't dash to his cause; instead they threw their weight behind Jeb Bush.

With him gone from the race, now the establishment is panicking that Trump might seal the deal and they think Rubio is the best chance of stopping him.

Christie just poked them in the eye.

Christie has said he didn't think Trump would be the Republican nominee.

Now he's just signed on to try to make him president.

Source: Al Jazeera