Pro-Kasich super PAC and Donald Trump go to war

A super PAC supporting John Kasich has begun attacking on Donald Trump, who has returned fire on Twitter and mobilized his legal team in response.

New Day for America, the super PAC supporting the Ohio governor, has begun running ads with Trump's face next to President Obama and Ben Carson while a narrator states, "On-the-job training for president does not work." The ad is reportedly just the beginning of a major anti-Trump crusade.

Trump responded quickly on Twitter and called Kasich a "total dud," "total failure," and "pathetic."

"I want to do negative ads on John Kasich, but he is so irrelevant to the race that I don't want to waste my money," Trump tweeted.

Instead of making ads, Trump threatened to sue Kasich on Twitter. Kasich cannot coordinate with the super PAC under federal regulations, so it's unclear how a lawsuit directed at the Ohio governor would stop the material that Trump finds objectionable.