"Any restriction on campaign spending violates the First amendment," Bernie Sanders tweeted Tuesday. | Getty Sanders: Gary Johnson would let the superrich 'buy elections'

Sen. Bernie Sanders on Tuesday said Libertarian Party nominee Gary Johnson’s campaign finance plan would let "billionaires” use their wealth to "buy elections."

Gary Johnson: “Any restriction on campaign spending violates the First amendment.”

Translation: Billionaires will be free to buy elections. Advertisement — Bernie Sanders (@BernieSanders) October 18, 2016





Sanders' tweet suggests an ongoing fear among Democrats and campaign officials that former Sanders supporters, as well as younger voters, could drop Hillary Clinton in favor of a third party candidate.

Johnson himself has begun to try to court Sanders supporters. On Thursday, his campaign released a new ad, titled "Where Bernie Sanders And Gary Johnson Agree." The two-minute campaign video featured Johnson highlighting what he and the Vermont senator had in common.

"You've probably seen Hillary Clinton and her media friends attacking me," Johnson said in the ad. "They're attacking because they're seeing the polls. The polls showing how many millennials and Independents are supporting the Johnson-Weld campaign."

Asked about Sanders' tweet, the Johnson campaign's communications director said: “It’s good that Sen. Sanders is listening.”