Few Democrats likely think that the Republican-controlled Senate will vote to remove President Trump from office if he is impeached. But they also have likely underestimated how a Republican-controlled Senate trial could be used to put Democrats on defense and exonerate the president.

The Senate has standing rules governing how impeachment trials are conducted. They do not establish many constraints on what the Senate can consider when sitting as a court. For example, there is no provision that the federal rules of evidence or criminal procedure be employed. Thus, the federal prohibition against hearsay evidence need not be adhered to if the Senate chooses to disregard it.

The fact that Supreme Court Chief Justice John G. Roberts Jr. would preside over that trial does not prevent the Senate from drafting its own bespoke procedures and rules. The Senate’s impeachment rules provide that Roberts’s evidentiary rulings can be subjected to a Senate vote and overturned according to the Senate’s standing rules. Presuming motions to overturn such rulings are handled according to Senate Standing Rule XX governing questions of order, a simple majority of the Senate would be sufficient to overrule the chief justice. In other words, Senate Majority Leader Mitch McConnell (R-Ky.), not Roberts, would be the de facto presiding officer if he so chooses.

It’s not hard to see why McConnell would choose to use the Republican majority in such a fashion. Suppose the president’s attorneys want to call Joe and Hunter Biden to testify about their activities in Ukraine. They would contend such testimony and investigation is necessary to exonerate the president as, under this theory, any acts he took to pressure the Ukrainian government to look into their activities would be a legitimate act to protect U.S. interests. The Republican base will also likely be whipped into a frenzy by the president’s media defenders to look into the Bidens’ alleged misdeeds. This type of defense probably wouldn’t be permitted in a regular criminal trial, but why would McConnell stand meekly to one side when the chance to effectively put the former vice president on trial presented itself?

The same political considerations apply to potential exculpatory evidence the president might submit. Imagine that his attorneys call a series of presidential appointees such as Secretary of State Mike Pompeo to the stand. The president’s counsel could ask them: “Did the president ever tell you that the aid to Ukraine would be withheld to force that country to investigate Joe Biden?” Normally, that could be considered hearsay evidence, as the answer is being presented to support the truth of the contention itself and Trump himself is not testifying to that statement. An aggressive McConnell, however, backed by a Senate majority, could vote to hear that evidence, opening the door to a parade of administration officials who would deny that the president ever told them to do anything illegal, all while Republicans refuse to call the president himself to the stand.

Nor will the whistleblower’s identity likely be protected if the Senate tries the case. Impeachment Rule XVII establishes that “witnesses shall be examined by one person on behalf of the party producing them, and then cross-examined by one person on the other side.” The whistleblower’s allegations are emerging as a central piece of evidence in the case so far. Presuming that continues, it defies belief that the president’s defense team would not require that person to testify under oath in front of the Senate. And it defies belief that a Republican majority hellbent on defending Trump would deny them that request. Rule XX provides that the Senate doors could be closed and testimony taken in secret if a majority so chooses, but why would Republicans agree to do that?

Everything we know about Trump suggests that he will direct his attorneys to mount a public and aggressive defense if the matter gets to the Senate. Unless the material unearthed by the House is so damaging as to cause mass Republican defections, everything we know about Republican voters is that they will want their team to back the president to the hilt. Everything we know about McConnell suggests he is a shrewd partisan operator who will not defy the president and the party’s base; instead, he will likely do everything in his power to embarrass, harass and defeat the Democrats. That combination means trouble for the Democrats.

The Senate trial of President Bill Clinton was a sedate affair even though Republicans controlled the Senate, because of Clinton’s sky-high approval ratings and Republican midterm losses. The Republican leadership rightly concluded that a long, bitter trial would only hurt the party. That’s not what’s going to happen in this election-year trial when members of the party base feel they themselves are on trial. Expect any Senate trial to be an explosive, highly partisan and bitter affair.

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