After a week of ups and downs in President Trump’s impeachment trial, the Senate is now barreling toward a showdown over whether to subpoena witnesses and documents.

Under special rules approved last week, senators will soon vote on whether to permit the parties to make motions to subpoena witnesses and documents to substantiate their claims presented in opening arguments. Republicans appear cautiously optimistic they can win that vote and bypass witnesses, and Minority Leader Chuck Schumer expects Democrats to lose it.

That senators are voting on whether to allow evidence to be subpoenaed at all is illustrative of how much these impeachment proceedings differ from past trials. Juxtaposing the earlier precedents and the present proceedings suggest that senators designed the rules governing Trump’s trial to create the appearance of a trial without actually holding a trial.

Senators modeled those rules on the special rules senators used in the 1999 impeachment trial of President Bill Clinton. Democrats and Republicans alike cited the Clinton procedures as a model for what the Senate should do for Trump's impeachment trial. Schumer referenced them in a letter he sent Majority Leader Mitch McConnell, outlining a proposal on how the trial should proceed. At the time, McConnell agreed with Schumer on the utility of following the Clinton model, stating , “the basic procedural framework of the Clinton impeachment trial served the Senate and the nation well, in my view…I still believe the Senate should try to follow the 1999 model.”

While convenient and orderly, the Clinton model encourages senators to think of the impeachment proceedings as consisting primarily in the parties’ opening statements. In doing so, they violate the Senate’s precedents , which stipulate, “The opening address of an impeachment trial is for the purpose of outlining what is expected to be proved. It is not for the purpose of introducing evidence to substantiate the charges.”

The Senate’s presiding officer followed those precedents in 1830 when, during the impeachment trial of District Court Judge James H. Peck, he “informed the managers that they might now proceed to substantiate their charge” after they finished delivering their opening statements. In 1905, the presiding officer reminded both parties not to introduce evidence in their opening remarks, stating that he believed “that the manager has a right to state what he expects to prove, but that he ought not to go further by citing any testimony which has been given by Judge Swayne on another occasion as the means by which he expects to prove it.”

Senators adopted this order of procedure to allow them to weigh the evidence presented by both sides in an adversarial, yet orderly, fashion instead of during the sequential and limited process represented by opening statements. Today, however, senators expect the parties to introduce evidence during opening statements delivered sequentially in a limited period. The parties are only allowed to present evidence to substantiate their claims if they persuade senators that an adversarial trial proceeding is warranted.

If the Senate votes to exclude witnesses and documents, it will represent a stark departure from its past practice. No witnesses testified in only three of the Senate’s 18 impeachment trials. In 1799 during its first trial, the Senate concluded that it lacked jurisdiction and dismissed the articles of impeachment against former Sen. William Blount. (While never formally adjudicated, the outcome of the Blount trial gave rise to the current understanding that the House can’t impeach members of Congress.)

In 1926, the Senate initially subpoenaed witnesses in the impeachment trial of District Court Judge George W. English. However, English resigned his judgeship as the trial was underway. Upon learning of English’s resignation, senators subsequently directed the Sergeant at Arms to notify those witnesses that they would not be required to testify. The Senate then dismissed the articles of impeachment upon request by the House managers.

Most recently, the Senate dismissed articles of impeachment against District Court Judge Samuel B. Kent, after being asked to do so by the House Managers. The proceedings lasted only one day. No witnesses were subpoenaed or testified.

In contrast, the Senate subpoenaed witnesses to testify in every other trial that proceeded to a final-verdict vote. Specifically, witnesses testified 15 times during the 1804 impeachment trial of District Court Judge John Pickering, 81 times during the 1805 trial of Associate Supreme Court Justice Samual Chase, 33 times during the 1830 Peck trial, and six times during the impeachment trial of District Court Judge West H. Humphreys. Witnesses testified 47 times during President Andrew Johnson’s 1868 impeachment trial and 45 times during the 1876 trial of William W. Belknap. Witnesses testified 47 times in the 1905 impeachment trial of District Court Judge Charles Swayne and 120 times during the trial of appellate justice Robert W. Archbald. Witnesses testified 46 times during District Court Judge Harold Lauderback’s 1933 trial and 57 times during District Court Judge Halstead L. Ritter’s trial in 1936.

More recently, witnesses testified 19 times during the trial of District Court Judge Harry E. Claiborne in 1986, 59 times in District Court Judge Alcee L. Hastings 1989 trial, and 10 times during the trial of District Court Judge Walter L. Nixon, Jr. that same year. Witnesses testified in depositions three times in President Bill Clinton's 1999 trial and three times during the 2010 trial of District Court Judge G. Thomas Porteous, Jr.

The Constitution empowers senators to make the rules for impeachment trials. The rules they ultimately adopt tell us a lot about how senators view the merits of the underlying impeachment.

Condensing the trial into opening statements and prohibiting evidence from being introduced to substantiate the parties’ claims suggests that senators want to create the appearance of a trial without actually having one.

James Wallner (@jiwallner) is a contributor to the Washington Examiner's Beltway Confidential blog. He is a senior fellow at the R Street Institute. Previously he was a Senate aide and a former group vice president for research at the Heritage Foundation.