GOP presidential frontrunner Donald Trump on Thursday threatened Sen. John McCain when he found out that the Party’s 2008 nominee agreed with Mitt Romney’s scathing attacks on Trump’s candidacy.

“I share the concerns about Donald Trump that my friend and former Republican nominee, Mitt Romney, described in his speech today,” McCain said.

When asked about McCain’s comments by CBS News’s Major Garrett after the Republican debate in Detroit on Thursday, Trump said McCain had better watch out. “That’s not nice, he has to be very careful,” Trump said. When asked why, he added, “He’ll find out.” Watch the clip:

 

This isn’t the first time Trump has tussled with McCain, as he got flack last year for saying the Arizona Republican isn’t a war hero. But Trump’s open threat has taken the feud a step further and the current favorite to become the GOP’s standard bearer is now in an open war with the previous two.

Threatening his party has also become something of a strategy for Trump, who previously announced that House Speaker Paul Ryan would “pay a big price” if he couldn’t along with a President Trump, and told owners of the Cubs, who’ve reportedly raised money to defeat Trump, that “[t]hey better be careful.”¬†Trump also threatened to sue Ted Cruz if he didn’t ease up on the mogul, and of course, he has a long history of threatening and initiating lawsuits in an attempt to get his way.