Ohio Gov. John Kasich warned on Sunday that the next commander-in-chief must not tolerate Russian aggression, sending a clear message to Donald Trump who has periodically commended Russian Prime Minister Vladimir Putin for his "strong" leadership.

"Putin, I think, is a thug. He wants to reconstitute the old Soviet Union and it's not going to happen," Kasich told CNN in a pre-taped interview. "The West needs to make it clear to Putin that we don't tolerate this behavior."

Kasich, who said he was "astounded" by Republicans' decision to remove language from the party platform about providing lethal defense assistance to Ukraine, said the U.S. can allow Putin to "save some face" and simultaneously take a hard line against his aggression in the Eastern European region.

"There's a way to do that," Kasich said. "You know, [Putin] has got to give back Crimea. Maybe he can have a base to the sea. But we cannot just show weakness in the face of a guy who's become very, very aggressive against the values of the West."

Trump has often told his supporters that he would work to develop a warmer relationship with Putin if he finds himself in the Oval Office.

"If we could get Russia to help us get rid of ISIS — if we could actually be friendly with Russia — wouldn't that be a good thing?" Trump said to Pennsylvania voters at a rally last week.