It could have been worse. Under intense pressure, including from members of his own party, Mr. McConnell slightly modified some of the most extreme provisions. The current version expands the time for the parties to present their cases to three days from two, and it allows the admission of evidence developed in the House — although the president may still object to evidence on any basis.

Mr. McConnell’s rules will weaken the power of impeachment as a deterrent to presidential misbehavior by, among other things, permitting the Senate to reject evidence and testimony that the House has already collected and that was not previously under dispute. In the worst case scenario, the rule could have the effect of allowing the Senate to dismiss the House’s evidence entirely. And these rules add new, unprecedented roadblocks to the Senate’s receiving witness testimony — including from witnesses who have already provided sworn statements to the House, some at great personal and professional cost.

Every impeachment trial in American history has heard from fact witnesses. And at least every impeachment trial in the last hundred years has heard fact testimony from new witnesses — those who have not previously provided evidence to the House of Representatives. Breaking from this ironclad precedent to protect President Trump, Mr. McConnell’s rules provide no assurance that the Senate will hear from witnesses or collect evidence.

Finally, Mr. McConnell’s proposal would allow senators to make a motion to dismiss at any time. Under a motion to dismiss, a defendant argues that, even if everything the prosecution alleges were true, the defendant still did nothing wrong. If such a motion were to pass before evidence is introduced, the Senate would effectively endorse the president’s conduct toward Ukraine without doing even the most basic fact-finding to determine the truth of the matter.

What happens next is not just for Mr. McConnell to decide. Each senator can uphold his or her constitutional duty and vote in the coming weeks, as they should have voted yesterday, to ensure that they have the information they need to decide whether the president solicited a foreign power’s assistance in his re-election and tried to prevent Congress from investigating that conduct, and whether that conduct necessitates his removal from office.