North Charleston, South Carolina (CNN) President Donald Trump suggested his supporters in South Carolina cast their primary ballots for Vermont Sen. Bernie Sanders during his rally Friday, the second straight Democratic primary in which he's suggested his supporters get involved.

"By the way, the Republicans allowed to vote in the -- are you ready?" he said to cheers from a crowd of supporters at a rally in North Charleston, South Carolina.

"Are you ready? Let's do it together," Trump said, going on to say that he "(assumed) this is OK from a campaign finance standpoint."

"First we have to figure out who would be the weakest candidate, and then if it's legal -- if it's permissible by law," the President said, railing against legal "booby traps" that he said punish Republicans disproportionately.

"Are we allowed to tell them who we would like you to vote for? So let's do a poll -- we do this for the fake news there," he said.

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