Former New York City Mayor Rudy Giuliani likes Rick Santorum on foreign policy and the economy but is opposed to his approach on social issues.

Appearing Friday on CNN, Giuliani gave Santorum a tepid endorsement.

“I like Rick's approach to foreign policy. I like Rick's approach to the economy. We have big differences on social issues. That's why I'm a moderate Republican. That's why I couldn't get nominated, if I ran,” Giuliani said.

Santorum is considered the GOP presidential field's most vocal opponent of gay rights, while Giuliani has previously said he supports civil unions for gay and lesbian couples.

“I'm concerned about how do we get back the Northeast as a voting block, when we seem to be not modern enough on social issues.”

“I think the biggest problem right now … I think the gay rights issue is a more current one right now. And I think beyond all the religious and social part of it, it makes the party look like it isn't a modern party. It doesn't understand the modern world,” Giuliani added. (Watch the exchange at CNN.)

(Related: Rick Santorum opposes civil unions for gay couples.)