Charles E. Schumer

Opinion contributor

After months of investigation, the House of Representatives has impeached President Donald Trump for abusing the powers of his office for personal gain and obstructing Congress in the investigation of those matters. As my colleagues return from the holiday recess, one question looms before the Senate: Will we now fulfill our duty to conduct a fair impeachment trial of the president and properly consider these extremely serious charges?

A fair trial is one that examines all the relevant facts and allows senators to make an informed decision based on those facts. And central to getting at the facts is summoning witnesses and a specific set of documents. Both presidential impeachment trials in our nation’s history featured witnesses. In fact, witnesses were called in almost every impeachment trial of any official in Senate history, according to information provided by the Senate Historical Office.

That is why I have called for the Senate to issue subpoenas for relevant documents and testimony from four current and former senior members of the administration with firsthand knowledge of the delay of military assistance to Ukraine and the efforts to pressure Ukraine to announce investigations of the president’s political rival. The witnesses are John Bolton, the president's former national security adviser; Mick Mulvaney, the president's acting chief of staff; Robert Blair, senior adviser to the chief of staff; and Michael Duffey, associate director for national security at the Office of Management and Budget (OMB).

Witnesses needed more than ever

Since Congress recessed for the holidays, several events have significantly bolstered the need to hear from these witnesses and obtain the related documents:

►On Dec. 20, the Center for Public Integrity obtained emails through a Freedom of Information Act request that showed that 91 minutes after President Trump’s July phone call with Ukrainian President Volodymyr Zelensky, Duffey asked the Defense Department to “hold off” on sending military aid to Ukraine.

►On Dec. 29, a New York Times report included several revelations about Mulvaney’s involvement in the delayed military assistance; about the effort by lawyers at the OMB, the Department of Justice and the White House to justify the delay; and about the alarm that the delay caused within the administration, particularly the Pentagon.

►Last Thursday, the Just Security blog cited a trove of newly unredacted emails that further exposed the serious concerns raised by Trump administration officials about the propriety and legality of the president’s decision to delay military assistance to Ukraine. One of those emails was from Duffey to the Pentagon comptroller: “Clear direction from POTUS (the president) to continue to hold.”

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►On Friday, despite a court order mandating that the White House comply with a FOIA request by The Times, the White House announced that it was withholding in full 20 emails between Blair and Duffey, in which they presumably discuss the freezing of military aid to Ukraine.

►And just this week, Bolton announced that he would comply with a Senate subpoena compelling his testimony. Given that his lawyers have stated he has new relevant information to share, we must hear his side of the story along with the other three witnesses.

Senate must shed light on the truth

Senate Majority Leader Mitch McConnell is very clear that he has no intention of conducting a fair trial that examines all of the evidence. But thankfully, the rules of the impeachment trial will be determined by a majority of all senators, not by the Republican leader alone. Right now, there are 47 Democrats and 53 Republicans in the Senate, which means four Republicans will make the difference between a trial that is fair, and a trial that serves as little more than a cover-up for the president’s misconduct.

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We are not asking for critics of the president to serve as witnesses in a trial. We are asking that the president’s men — his top advisers — tell their story, and for the Senate to have access to the documents that will shed light on the truth.

The bottom line is simple: There was an exceedingly strong case to call witnesses and request documents before the Senate recessed for the Christmas break. Since then, that case has gotten even stronger.

Sen. Charles E. Schumer, D-N.Y., is the Senate minority leader. Follow him on Twitter: @SenSchumer