Presidential candidate Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE rejected presumptive GOP presidential nominee Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE's characterization of the Democratic primary system as "rigged" but did give his own harsh assessment.

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"Well, I've been very touched by Donald Trump's love for me. But John, you know, with all due respect, I think there may be some aspect of this which he thinks will advantage himself," he told host John Dickerson on CBS News's "Face the Nation" in an interview to air Sunday.

On Thursday, Trump gave "Jimmy Kimmel Live" host Jimmy Kimmel a question to ask Sanders during his appearance on the show. He asked if Sanders would launch a third-party campaign because “both primary systems are rigged, but in particular the Democrats' ridiculous system of superdelegates."

"I wouldn't use the word 'rigged,' because we knew what the rules were," Sanders said on "Face the Nation." "But what is really dumb is that you have closed primaries, like in New York State, where 3 million people who were Democrats or Republicans could not participate. Where you have a situation where over 400 superdelegates came on board Clinton's campaign before anybody else was in the race, eight months before the first vote was cast. That's not rigged. I think it's just a dumb process, which has certainly disadvantaged our campaign."

Sanders and his supporters have protested the Democratic nomination process, especially the influential role of superdelegates and the limited amount of primaries open to independent voters.

Sanders also continued to hit Trump for backing out of a proposed debate, saying he has flip-flopped on the idea.

"I think that is who Donald Trump is, and I think the American people should be very concerned about somebody who keeps changing his mind not only on this debate, but on virtually every issue he's been asked about," he said.