Former Republican presidential candidate Rick Santorum endorsed Donald Trump in an interview with Fox News host Greta Van Susteren Tuesday night.

"I'm 100 percent. That's endorse," the former Pennsylvania senator said when asked if he was backing the presumptive GOP nominee.

Santorum had endorsed Sen. Marco Rubio in February before the Florida politician dropped out of the primary contest in March.

Santorum said he was moved to back the Manhattan businessman after talking with him last week.

"The most important issue is preserving the Constitution of this country and a liberal Supreme Court will destroy it," Santorum said.

Trump released a list of 11 Supreme Court justice nominees last week, which was generally met by praise from conservatives.

"Donald Trump still has an opportunity to cast the right lot and win this election," Santorum added. "He's been very real, laying it all out on the line talking about things most politicians have not talked about."

Trump tweeted his appreciation shortly after the announcement.