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John McCain.
"I promise you that we will be united against any Supreme Court nominee that Hillary Clinton, if she were president, would put up," McCain told 1210 WPHT radio in Philadelphia, first reported by CNN. "I promise you."
He added that the Republicans need to retain control of the Senate for that exact reason, and that reelecting Sen. Pat Toomey of Pennsylvania was key in doing so.
"This is where we need the majority and Pat Toomey is probably as articulate and effective on the floor of the Senate as anyone I have encountered," he said.
McCain recently unendorsed Republican nominee Donald Trump after a leaked 2005 tape of the Manhattan billionaire making crude sexual remarks about women was made public earlier this month. The Arizona Republican likened Trump's comments to "boasts about sexual assault."
In his WPHT interview, McCain said he wasn't sure if Trump would be a better president when solely looking at the Supreme Court. Trump released an extended list last month of 21 potential justices he'd nominate, extending a shorter list he released earlier this year.
Senate Republicans have blocked President Barack Obama's nomination of Judge Merrick Garland to the court. Obama nominated Garland in March to fill the seat vacated by Justice Antonin Scalia.