Former GOP presidential candidate John Kasich said Monday that he will not launch a third-party White House bid, ending speculation about him re-entering the race to challenge Donald Trump.

"I'm not going to do that," the Ohio governor said in a CNN interview, his first since leaving the GOP primary in early May.

Kasich declined to confirm reports that 2012 GOP presidential nominee Mitt Romney tried to recruit him to be the party’s conservative alternative to Trump, the presumptive Republican nominee, saying only that “somebody” called him.

“A third party candidacy would be viewed as kind of a silly thing,” he said. “And I don't think it's appropriate. I just don't think it would be the right thing to do."

Any candidate would have a difficult time mounting a credible independent bid, with no campaign or fundraising infrastructure in place and time expiring to get on state ballots.

Kasich, a former congressman, was popular among Washington conservatives, sometimes seen as the level-headed alternative to Trump.

However, he won only his home-state primary and with Texas GOP Sen. Ted Cruz dropped out of the race after Trump trounced them in the May 3 Indiana primary.

"I gave it my best where I am,” Kasich said Monday. “I just think running third party doesn't feel right. I think it's not constructive."