Washington (CNN) Senate Minority Leader Chuck Schumer on Tuesday said there are 10 to 12 Senate Republicans "who know in their hearts" there should be witnesses and documents in President Donald Trump's Senate impeachment trial ahead of a looming vote later this week.

The comments come after Senate Majority Leader Mitch McConnell, a Kentucky Republican, told his conference earlier Tuesday that the votes to shut out witnesses aren't there yet. But Senate Republicans are growing confident they can defeat a witness vote after weathering the fallout of revelations from former national security adviser John Bolton's draft book manuscript that Trump had told him US security assistance to Ukraine was conditioned on investigations into Democrats, including the Bidens.

"There are 10 to 12 Republicans who have never said a bad word about witnesses or documents, who know in their hearts it's the right thing to do. But they have to weigh that against the pressure, the twisting of arms, that Donald Trump and Mitch McConnell will put on them," Schumer told CNN's Erin Burnett on "OutFront."

"So I think it's up in the air right now," the New York Democrat added. "Are we feeling better today than we did a few days ago? Absolutely."

The Bolton developments jerked the conversation over witnesses in the Senate into a new direction. But several GOP senators and aides said the White House presentations on Monday had helped halt a potential scenario in which a handful Republicans moved sharply toward wanting to hear from witnesses.

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