THE IMPEACHMENT of President Trump returned to center stage in Washington after a four-week interregnum that might have cost it momentum and public attention but that also improved the prospects for a credible Senate trial. Even before the House transmitted two articles of impeachment across the Capitol, President Trump’s proposal that the Senate dismiss the charges without a hearing was rejected by Senate Majority Leader Mitch McConnell (R-Ky.), who said he didn’t have the votes for it. Meanwhile, what has been a steady trickle of new evidence in the case continued with the disclosure of texts and emails strengthening the case that Mr. Trump attempted to extort political favors from Ukraine.

The seven impeachment managers named by the House will have a solid case to lay out next week concerning Mr. Trump’s use of his official powers to induce Ukraine’s president to announce an investigation of Joe Biden and of conspiracy theories about Ukrainian interference in the 2016 election. The second article, on Mr. Trump’s obstruction of Congress, is self-evident: The president has improperly blocked aides from responding to congressional subpoenas seeking further testimony and documents.

The test of the Senate’s integrity will come with a promised vote on whether to call for the witnesses and documents impeded by Mr. Trump’s stonewalling. Doing so is essential for two reasons. First, the testimony of top aides such as former national security adviser John Bolton could confirm — or perhaps undercut — the case that Mr. Trump withheld military aid and a White House meeting from Ukrainian President Volodymyr Zelensky to force him to help Mr. Trump’s reelection effort. White House documents on the aid suspension could be even more telling.

In addition, Congress must demonstrate that a president cannot simply reject its oversight, especially on a matter as grave as impeachment. Mr. Bolton has said he would testify if the Senate calls him, and he has a book forthcoming. Could it be that the Senate will refuse to hear a firsthand account that could soon be in the public domain?

Mr. Trump and Mr. McConnell are advancing the cynical argument that it was up to the House to gather evidence, and the Senate should not be tasked with filling gaps. But the gaps exist because of Mr. Trump’s obstruction, a stonewall that the House probably could not have overcome without months of court proceedings that, during an election year, could have made impeachment a practical impossibility. Mr. Trump has said he will invoke executive privilege to curtail testimony by Mr. Bolton and others. That should only incentivize senators: If the president had nothing to fear from truthful testimony, he would not go to such lengths.

Mr. McConnell, who has already promised to violate his oath by not remaining impartial, will do his best to cut the trial short. So it is encouraging that several Republican senators, including Mitt Romney (Utah) and Susan Collins (Maine), are suggesting they will at least consider voting for witnesses. In truth, the question should not be a hard one: In a matter as grave as impeachment, a trial that is seen by the public as thorough, rather than partisan, is essential. Still, given the polarized political climate, and Mr. Trump’s demands for absolute fealty from his party, even common sense can look courageous.

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