Joe Biden warned a group of high-dollar donors that rabid online Bernie Sanders fans will try to make the 2020 Democratic primary nasty, in the process mangling the “Bernie bros” nickname bestowed on the Vermont senator’s most vocal male supporters.

“What we can’t let happen is let this primary become a negative bloodbath. I know I’m going to get a lot of suggestions on how to respond to what I suspect will be an increasingly negative campaign that the Bernie brothers will run,” the former vice president said Friday by phone to a group of about 130 donors in the Washington, D.C., suburb of Bethesda, Maryland, according to a pool report. He lengthened the common “Bernie bros” nickname for online fans of the Vermont senator.