THE HOUSE of Representatives’ impeachment of President Trump on Wednesday was proper and necessary. Mr. Trump withheld a White House meeting and U.S. military aid in an attempt to force Ukraine’s president to aid his reelection campaign. This was a gross abuse of his office that Congress could not allow to go unpunished. Nor could it acquiesce in Mr. Trump’s stonewalling a constitutionally authorized inquiry with a blanket refusal to cooperate with lawful subpoenas for documents and the testimony of senior aides.

Whether or not a Senate trial leads to his conviction and removal from office, Mr. Trump has deservedly suffered an indictment imposed on only two previous American presidents. The two articles of impeachment reinforce essential, and what should be self-evident, norms of our democracy: that presidents cannot use their powers to extort political favors from foreign governments, and that they cannot comprehensively reject congressional checks. That Mr. Trump denied all wrongdoing made the House action only more necessary.

The vote to impeach was difficult and politically risky for many Democrats. Some who won seats only last year in swing districts carried by Mr. Trump acknowledged they might be endangering their careers. Representatives such as Abigail Spanberger (D-Va.) and Elissa Slotkin (D-Mich.) deserve credit for putting their allegiance to the Constitution over political calculations.

The absence of Republican votes for impeachment, in contrast, revealed the party’s fundamental corruption by Mr. Trump. Though some GOP representatives have acknowledged that the president’s actions were improper, none were principled enough to break with him. Many cravenly adopted Mr. Trump’s indefensible position that there was nothing wrong with the pressure campaign he directed at Ukraine’s neophyte president.

Mr. Trump himself set the tone for his defense with a ranting, falsehood-packed letter to House Speaker Nancy Pelosi (D-Calif.) in which he repeated the proven lie that Joe Biden sought the dismissal of a Ukrainian prosecutor to protect his son and the Ukrainian gas company that employed him. Even as he faced impeachment, Mr. Trump was still pursuing the primary aim of his extortion of the Ukrainians — to smear his potential 2020 opponent.

The president’s refusal to acknowledge error and the likelihood that he will continue to violate democratic norms make imperative a full and impartial Senate trial of the charges against him. Unfortunately, Majority Leader Mitch McConnell (R-Ky.) has made clear that he has no intention of allowing such a trial. Bluntly declaring that “I’m not an impartial juror,” Mr. McConnell indicated Tuesday that he will seek a vote of dismissal on the charges against Mr. Trump before allowing any testimony — even though crucial witnesses to Mr. Trump’s behavior, including former national security adviser John Bolton and acting chief of staff Mick Mulvaney, have never been heard from.

That ought not to be acceptable to Republicans who have criticized Mr. Trump’s behavior, such as Sen. Mitt Romney (Utah), or those who have said they will take their duty as jurors seriously, such as Sen. Susan Collins (Maine). They should tell Mr. McConnell they cannot judge Mr. Trump’s guilt or innocence until they have heard those firsthand accounts Mr. Trump has tried to suppress. Republicans have accused Democrats of cheapening the impeachment remedy, but if they fail to hold a credible trial of the serious charges against Mr. Trump, it is they who will damage our constitutional system.

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