House Speaker Nancy Pelosi (D-Calif.) is withholding two articles of impeachment from the Senate, pending assurance that the Republican leader of that body, Sen. Mitch McConnell (Ky.), will agree to a full and fair trial of the House’s charges against President Trump. Whatever else may be said about the speaker’s move, and however long her holdout lasts, it has certainly taken advantage of some inevitable holiday-season downtime to focus attention on the Senate’s role in the process. So far, that has meant much-needed discussion of Mr. McConnell’s obvious — and obviously political — intention to go through the motions of a trial on the way to an acquittal.

Now fresh reporting from the New York Times has emerged to strengthen the Democrats’ minimum condition of a real trial: The Senate must seek witness testimony from key players in Mr. Trump’s attempt to strong-arm Ukraine into announcing an investigation of his political rival, former vice president Joe Biden, using congressionally appropriated military aid and promises of a White House visit as leverage. The Times reports, based in part on previously undisclosed emails, that acting White House chief of staff Mick Mulvaney tried to freeze the military aid on Mr. Trump’s behalf as early as June, prompting puzzlement and backlash within the administration — to the extent that Defense Secretary Mark T. Esper, Secretary of State Mike Pompeo and then-national security adviser John Bolton convened a White House meeting with Mr. Trump to urge release of the aid. Mr. Trump, apparently fixated on the idea that Ukraine had tried to defeat him in 2016, balked, asserting, contrary to Defense Department certifications, that Ukraine was hopelessly corrupt.

More than ever, therefore, the Senate and the public need to hear from Mr. Mulvaney and Mr. Bolton, the latter of whom made an unsuccessful individual plea to release the Ukraine aid on Aug. 16, according to the Times. Their testimony, and that of Mr. Mulvaney’s top aide, Robert B. Blair, and Office of Management and Budget official Michael Duffey, has always been crucial, which is why House impeachment investigators initially sought it and — undoubtedly — why the White House refused to allow it.

Certainly, the House’s demand that the Senate call these witnesses is undercut to some extent by its own failure to persist in trying to compel their testimony, hoping to avoid getting bogged down in an extended court fight with the administration. Yet the question of whether the Senate should exercise its subpoena power to obtain relevant testimony is a separate issue, politically, legally and morally. A Senate leader sincerely interested in operating as head of an independent branch of government would have left no doubt that he intended to do so. Mr. McConnell has done the opposite, giving rise to Ms. Pelosi’s very legitimate concerns.

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