But Senate Republicans, led by Sens. John Cornyn of Texas and David Perdue of Georgia, countered that the House managers should proceed with the evidence they used to impeach Trump in the House.

And Republicans echoed the White House line that the House impeachment hearings violated Trump’s right to due process, despite the fact that the president refused to allow his lawyers to participate in those sessions.

“I find it curious that Chairman Nadler of the Judiciary Committee called this a ‘rock-solid’ case,” Cornyn said on CBS’ “Face The Nation. “But if the House isn’t prepared to go forward with the evidence that they produced in the impeachment inquiry, maybe they ought to withdraw the articles of impeachment and start over again. This isn’t the Senate's responsibility to make the case.”

“This, to me, seems to undermine or indicate that they’re getting cold feet or have a lack of confidence in what they’ve done so far,” Cornyn added.

Perdue said on NBC’s “Meet The Press”: “Remember, this week is going to be the first time America gets to hear President Trump’s defense. He hasn’t had an opportunity to do that yet. It’s clear the president did not have due process in the House. Now, for the first time, we’ll have due process in the Senate.”

A number of Senate Republicans, including Cornyn, have called for former Vice President Joe Biden’s son Hunter to be deposed if Bolton testifies in the case. Trump allies are calling this “witness reciprocity,” and McConnell appears open to their demand, according to GOP aides familiar with these discussions.

Yet Nadler, Schiff and the other House managers maintain that Hunter Biden is not germane to the case, since he cannot speak to the underlying issue of whether Trump improperly withheld U.S. aid to Ukraine contingent on officials there announcing an investigation into the Bidens.

“And this whole controversy about whether there should be witnesses is just really a question of, does the Senate want to have a fair trial … or are they part of the cover-up of the president?” Nadler said on “Face The Nation.“ “Any Republican senator who says there should be no witnesses or even that witnesses should be negotiated is part of the cover-up.”

So far, only three Senate Republicans — Susan Collins of Maine, Lisa Murkowski of Alaska and Mitt Romney of Utah — have publicly declared that they’re open to hearing from additional witnesses, including Bolton. But in order for that to occur, at least one more Senate Republican would have to withstand Trump’s pressure and cross the aisle to vote with the 47 Senate Democrats.