WASHINGTON — In the end, the impeachment calculation nearly all Senate Republicans are making is fairly simple: They would rather look as if they ignored relevant evidence than plunge the Senate into an unpredictable, open-ended inquiry that would anger President Trump and court political peril.

As Republicans lined up on Wednesday behind blocking witnesses in the trial, their reasoning reflected the worry that allowing testimony by John R. Bolton, the former national security adviser whose unpublished manuscript contradicts a central part of Mr. Trump’s impeachment defense, would undoubtedly lead to a cascade of other witnesses. They in turn could provide more damaging disclosures and tie up the Senate indefinitely, when the ultimate verdict — an acquittal of the president — is not in doubt.

“For the sake of argument, one could assume everything attributable to John Bolton is accurate, and still the House would fall well below the standards to remove a president from office,” said Senator Lindsey Graham, Republican of South Carolina.

Republicans have offered myriad rationales for refusing new testimony: Gathering it was the House’s job. Calling more witnesses would lead to prolonged court fights over executive privilege. They had heard more than enough evidence to reach a verdict. There was not enough evidence to show they needed more information. Allowing the House to force the Senate into a drawn-out impeachment trial would set a dangerous institutional precedent. In essence, during what they hoped would be the final hours of Mr. Trump’s trial, Senate Republicans were constructing a permission structure for not trying to get to the bottom of what happened, with the hope that voters would find their explanations satisfactory and reasonable.