Since its founding in 1873 as a “school farm” for disadvantaged city youth, the McDonogh School has made honesty and integrity a core part of its mission. That is as true today — as 13-year-old students are required to sign a pledge promising to “tell the truth in all situations, [and] present work that is truly their own” — as it was in 1966, when the graduating class included a Baltimore firefighter’s son named John Robert Bolton.

Standing in contrast is Donald John Trump, the son of privilege who attended the New York Military Academy (Class of 1964), where he lost his rank of captain his senior year (an action classmates attributed to a hazing incident that Mr. Trump has denied). When it’s time to decide who is telling the truth, do you go with the Yale Heights native who went on to become Mr. Trump’s national security adviser or the serial liar who, according to The Washington Post’s most recent tally, has made 16,241 false or misleading statements since taking office?

Is that a trick question? It really isn’t. Yet the sad reality is that the Republican-controlled U.S. Senate may soon vote not to hear from Mr. Bolton, or anyone else, as they move forward with President Trump’s impeachment trial. Given the weekend’s revelation from The New York Times that Mr. Bolton’s working book manuscript details how Mr. Trump asked him to continue freezing $391 million in aid to Ukraine until officials in that country agreed to investigate Democrats, including former vice president Joe Biden and his son Hunter, that level of disinterest boggles the mind. Essentially, Mr. Bolton stands as a firsthand witness to the allegations at the center of the impeachment case. And he’s ready to spill all — to his readers. How can the Senate possibly turn this opportunity down?

Indeed, how could any Republican loyal to President Trump not want to question Mr. Bolton? Shortly after midnight Monday, Mr. Trump was on Twitter denying he ever told Mr. Bolton that the aid was conditioned to those investigations. “If John Bolton said this,” the president tweeted, “it was only to sell a book.” He also retweeted criticisms from his usual Fox News allies questioning the timing of the Bolton scoop. But to that, one can only say: So what? So what if the Bolton manuscript was leaked by, gasp, a liberal? If it’s not true, shouldn’t Republicans relish the opportunity to put the former national security adviser under oath? Shouldn’t the administration seek this chance for exoneration? Would it really be preferable to leave these allegations unexplored, unchallenged, uncontradicted?

Make no mistake, we don’t endorse Mr. Bolton’s politics, his disdain for the United Nations and other international organizations, his hawkishness, his abrasiveness, his contempt for diplomacy. But at least such views are consistent. His dark perspective on geopolitics appears little changed (although how he held his tongue during President Trump’s North Korea outreach remains a mystery). But whatever his pattern of behavior from his days touting Barry Goldwater to his McDonogh classmates to his long-held antipathy toward Iran, it doesn’t appear to involve blatant lies. Are Americans supposed to trust the word of a president who can’t bring himself to be truthful about an Alabama hurricane forecast, the lack of a link between windmill noise and cancer or the place of his father’s birth (It was New York, not Germany, as Mr. Trump has repeatedly claimed)? And those are just some of the blandest of Mr. Trump’s epic inventory of prevarications.

Senate Majority Leader Mitch McConnell may want to wrap up his kangaroo court as expeditiously as possible. But refusing to hear the testimony of Mr. Bolton, as well as other firsthand witnesses like Mick Mulvaney, not only exposes the phoniness of these deliberations, it’s contrary to his party’s own interests, at least if they are serious about restoring credibility in time for the 2020 elections. Whether House Democrats should have subpoenaed Mr. Bolton during their impeachment proceedings — and thus found themselves locked in a court battle over executive privilege — is irrelevant. The nation is paying sufficient attention to understand that something fishy is going on right now.

If, as the Trump legal team claims, the Democrats have produced insufficient evidence, how can Senate Republicans not want to have chat with a McDonogh grad who was in a position to know for certain? Isn’t that what any impartial judge would want?