Republicans "can't just simply stonewall" nominees to the Supreme Court even if the president making the choice is Democrat Hillary Clinton, says the GOP chairman of the Judiciary Committee in a reaffirmation of the Senate's advise-and-consent role on judicial picks.

Iowa Sen. Chuck Grassley's comments on Tuesday was a response to fellow Republican Sen. John McCain, who a day earlier vowed that Republicans would unite against any nominee Clinton puts forward if she becomes president. That unprecedented pledge raised the possibility that the Supreme Court would have to operate for four years of a Clinton term with one or more vacancies, rather than nine justices.

The court has had one vacancy for months since the death of Justice Antonin Scalia in February. Republicans have refused to consider President Barack Obama's nomination of Merrick Garland, arguing that the next president should fill the opening.

"I think we have a responsibility to very definitely vet — if you want to use the word vet — whoever nominee that person puts forward," Grassley told radio reporters in Iowa. "We have the same responsibility for (Donald) Trump. We know more the type of people Trump would nominate because he's listed 20. They fall into the category of strict constructionists. As I heard about Hillary on the last debate, the type of people she's going to appoint, I would say they're judicial activists."

He added that the new president should make the choice and "if that new president happens to be Hillary. We can't just simply stonewall."