Sen. John Fetterman said Saturday that elevating Sen. Chris Van Hollen to Senate Democratic leader would be troubling, raising the possibility that the Pennsylvania Democrat could leave his party over the choice.

Asked on Fox News whether there was a red line that would cause him to leave, Fetterman pointed to Van Hollen’s bid to replace Chuck Schumer, citing the Maryland senator’s position on Israel. Pressed on whether he would leave the party, Fetterman said he didn’t know what that would look like, adding that he remains a Democrat. Watch the exchange.

Fox News interview. The leadership exchange begins around 5:09. Watch directly on Fox News.

Van Hollen announced his challenge to Schumer last week as Democrats prepare to take the Senate majority in January. Fetterman is backing Schumer. He has not announced a party switch.