PHILADELPHIA -- New Jersey Gov. Chris Christie scored a small -- and likely unwanted -- part in the opening night of the Democratic National Convention on Monday.

As the Democratic Party laid out its platform against Republican nominee Donald Trump, it used a television clip of Christie, a one-time Trump rival turned GOP nominee backer, to help try to build its case against the billionaire businessmen.

A clip of the governor appeared in the convention arena in Philadelphia that showed Christie criticizing Trump during the Republican primary for apparently mocking a journalist with a muscular disability. The clip of Christie was part of a short video that criticized Trump for gestures he made on the trail last year when he mimicked a reporter while making sudden body motions.

Critics charged Trump was mocking the reporter's disability.

At the time, Christie criticized Trump for his actions.

"He shouldn't be making fun of people's disabilities. It's just not worthy of someone running for president of the United States," Christie said in December 2015, which was rehashed during the convention.

Christie would go on to endorse Trump less than less three months after he made that comment.

The governor backed Trump weeks after he dropped out of the campaign after a poor showing in the New Hampshire primary. He now heads Trump's transition team and was among the top contenders to be selected as Trump's running mate.

Matt Arco may be reached at marco@njadvancemedia.com. Follow him on Twitter @MatthewArco or on Facebook. Follow NJ.com Politics on Facebook.