But some conservative activists, eager to unearth any political advantage, are encouraging Republicans to vote this weekend anyway — for a Democrat.

If they can stomach it, backers said, Republicans should take advantage of South Carolina’s open primary system, which allows voters to cast a ballot without registering with a party. Multiple organized efforts are underway, with one going so far as to quote-unquote endorse Senator Bernie Sanders of Vermont, who state polls show is closely trailing former Vice President Joseph R. Biden Jr…

But they said they were motivated by a larger mission: For years, conservative activists have sought to close South Carolina’s open primaries, and this election offered ideal circumstances for creating a live demonstration of the flaws they see in the system.

“We were also looking for a way to get Democrats to support the cause of closing the primary,” said Christopher Sullivan, a longtime critic of the open primary and leader of the Conservative Defense Fund, a small advocacy outfit in South Carolina. “And this is a way of showing them what’s good for the goose is good for the gander.”