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 aide Paul Manafort said in an interview Sunday that the presumptive GOP presidential nominee is not scared to debate Democratic 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.

"He's not afraid to face off with Bernie Sanders," Manafort said on ABC's "This Week."

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"He's the Republican nominee now. The question should be why is [Democratic presidential front-runner] Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE afraid to debate Bernie Sanders."

Manafort said Clinton is the one running against Sanders in the Democratic primary.

"The point that Mr. Trump was trying to make was the Democratic system is rigged; it's been rigged from the beginning for Hillary Clinton," he said.

Sanders has been winning primaries and losing delegates, he continued.

"Now you've got California, the state where Clinton thought she was going to win. The race has gotten very close. Bernie Sanders wants to debate Hillary Clinton. Bernie Sanders should have that chance," Manafort said.

"Mr. Trump was saying debates are important. He would have debated Bernie Sanders if Bernie Sanders were the nominee, but Hillary Clinton should be debating him today."

Trump had originally said he wanted to debate Sanders, but then said it would be "inappropriate" to debate the Vermont senator. Trump called Sanders a "second-place finisher."

Manafort said his candidate would debate the Democratic nominee.