WASHINGTON – Ever since Donald Trump became the presumptive Republican nominee for president, he’s been focusing most of his attacks on his Democratic rival, Hillary Clinton, and imploring her challenger to enter the race as an Independent.

“Bernie Sanders is being treated very badly by the Dems,” Trump tweeted on Monday. “The system is rigged against him. He should run as an independent! Run Bernie, run.”

Bernie Sanders, an Independent senator from Vermont, has had an impressive rise this election cycle, mounting a surprisingly tough challenge to Clinton. Although he’s the longest-serving Independent member of Congress, he’s been running for the Democratic nomination for president.

Trump has been pushing this message of Sanders being treated unfairly by the Democratic party for weeks.

“Bernie Sanders has been treated terribly by the Democrats—both with delegates & otherwise,”Trump tweeted in late April. “He should show them, and run as an Independent!”

Despite Trump’s advocacy on Sanders’ behalf, the real-estate mogul has dubbed him “crazy Bernie” in other tweets.

Sanders running as an Independent against Trump and Clinton would largely benefit Trump. Sanders, who has built up a significant amount of popular support, could win voters who might otherwise support Clinton.

Trump more or less acknowledged this himself in tweets earlier this month:

“Get ready for November,” Trump tweeted on May 11. “Crooked Hillary, who is looking very bad against Crazy Bernie, will lose!”

He said in a subsequent tweet: “I don’t want to hit Crazy Bernie Sanders too hard yet because I love watching what he is doing to Crooked Hillary. His time will come!”

Both Trump and Sanders have railed on their respective party’s nominating processes, calling them “rigged” to favor establishment candidates.