THE IMPEACHMENT of President Trump, which is due to come to a vote in the House of Representatives this week, ought to inspire the utmost seriousness and patriotism from members of Congress. Instead, the process is in danger of being devalued into a predictably partisan House vote, followed by a truncated and inadequate Senate trial. Crucial witnesses to Mr. Trump’s alleged abuses of power may never be heard; the president’s flagrant stonewalling of congressional authority may be allowed to stand. Many Americans seem to be tuning out, having anticipated just such an outcome.

Some representatives are exhibiting courage, including several Democrats in swing districts who have announced they will vote for impeachment even at the risk of losing their seats. “There are some decisions in life that have to be made based on what you know in your bones is right,” wrote Rep. Elissa Slotkin (D-Mich.) in an op-ed published Monday. A few Republicans, such as Rep. Will Hurd (R-Tex.), have at least acknowledged that Mr. Trump’s actions toward Ukraine were improper, while questioning whether they justify removal from office.

Yet many Republicans appear intent on refusing to engage seriously with the articles of impeachment and the evidence behind them. The bald declaration by Sen. Lindsey O. Graham (R-S.C.) — “I’m not trying to pretend to be a fair juror here” — appears to apply to a number in his party, including Senate Majority Leader Mitch McConnell (R-Ky.), who has pledged to coordinate closely with the White House on a trial whose outcome — acquittal — he has already declared. He and his caucus should consider the damage they are doing to the Constitution for the sake of political expediency.

We have argued that the evidence gathered by the House justifies the impeachment of Mr. Trump for seeking to force Ukraine to announce politicized investigations of Joe Biden and the 2016 U.S. election, and for obstructing Congress’s investigation. But the speed of the inquiry — Democrats, too, have been overly eager to dispose of the matter — and the White House’s obstruction have left important questions unanswered, and a probably persuadable part of the public unconvinced.

The uncertainty has been compounded by the implacable refusal of Republicans on the House Judiciary Committee to address the established facts of the months-long pressure campaign conducted by Mr. Trump and his personal lawyer, Rudolph W. Giuliani. Instead, they have offered spurious complaints about process and manifestly false alternative narratives — including the Russian-concocted allegation that Ukraine and not Russia intervened in the 2016 election.

We can hope that more rational Republican voices will be heard during the House floor debate. But more likely it will be up to Senate Republicans to ensure that Mr. Trump is fully and fairly judged. They can do so by rejecting the rushed trial Mr. McConnell appears to be contemplating and supporting the summoning of key witnesses whose testimony has been missing.

In a letter to Mr. McConnell on Sunday, Senate Minority Leader Charles E. Schumer (D-N.Y.) proposed four: acting chief of staff Mick Mulvaney; former national security adviser John Bolton; Robert Blair, an adviser to Mr. Mulvaney; and Michael Duffey, an official at the Office of Management and Budget. Their testimony might not change the outcome of a trial, but it would at least offer the assurance that the allegations against Mr. Trump had been fully explored.

Republicans as well as Democrats must invest the impeachment process with the gravity it should have as the ultimate constitutional check on presidential abuses. The alternative is to open the way for Mr. Trump to abuse his office with impunity.

Read more: