NEW EVIDENCE continues to emerge, yet Republican senators, led by Senate Majority Leader Mitch McConnell (R-Ky.), seem no more interested in conducting a serious inquiry into President Trump’s corrupt Ukraine pressure campaign.

“It’s the Senate’s turn now — to render sober judgment as the framers envisioned,” Mr. McConnell declared Friday, even as he promised nothing of the sort, dismissing calls for the Senate to demand still-hidden documents and the appearance of key witnesses as it considers removing Mr. Trump from office. Sen. Susan Collins (R-Maine), while professing to be “open to witnesses,” said it is “premature to decide who should be called until we see the evidence that is presented.”

The flaw in that premise is that the White House is blocking disclosure of critical evidence. The material already in the record is damning, but questions remain about the extent to which Mr. Trump explicitly linked the release of Ukrainian security aid to the country’s willingness to conduct phony investigations of Trump rival Joe Biden. It is no mystery whose accounts are missing: Acting White House chief of staff Mick Mulvaney, Secretary of State Mike Pompeo and former national security adviser John Bolton have a duty to testify.

And there is Office of Management and Budget (OMB) official Michael Duffey, a player in the Ukraine drama whose role is becoming clearer. The website Just Security said Thursday that it had obtained unredacted communications between Defense Department officials and their counterparts at OMB, including Mr. Duffey. These emails appear to show that a hold on Ukraine aid came at the “clear direction from POTUS” and that staff became increasingly alarmed about the legality of refusing to disburse money Congress had appropriated. An investigation into how executive branch officials understood and raised concerns about the Ukraine aid blockage is a must.

The memos also underscore the significance of the second article of impeachment, focused on Mr. Trump’s obstruction of the impeachment process.

“As part of our impeachment inquiry, the House subpoenaed these very documents,” House Intelligence Committee Chairman Adam B. Schiff (D-Calif.) said Thursday. “From their deeply incriminating character, we can now see why they were concealed: They directly corroborate witnesses who testified that military aid to Ukraine was withheld at the direction of the President and that the White House was informed doing so may violate the law. The Administration did not want Congress to find out why.”

Unlike in 1998, during the impeachment of President Bill Clinton, Congress lacks a voluminous report from Justice Department investigators; lawmakers have had to assemble their own record. Also unlike in 1998, the White House has refused to cooperate in any way with the inquiry. The Senate should have the self-respect to ask for needed information and take the president to court if he refuses to turn it over.

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