How extraordinary, then, that some Republican Senators are now pushing to deliver precisely what Ukraine refused to provide. As Mr. Trump’s impeachment trial begins, Republicans like Rand Paul have insisted that, if any witnesses testify, those witnesses include Hunter Biden.

Mr. Trump himself has gone further, demanding that Hunter and Joe Biden testify. But to have any Biden testify would give Mr. Trump exactly what he sought from Ukraine. After all, neither Biden is actually a relevant witness: neither has insight into whether Mr. Trump committed the impeachable offenses of which he stands accused. That type of relevant witness — which would include John Bolton and Mick Mulvaney — is precisely the type that the president and Senate allies like Mr. Paul and Mitch McConnell, the majority leader, are resisting.

The Bidens, by contrast, know nothing and therefore have nothing to say about what Mr. Trump ordered, why he did it, or whether it constitutes a high crime or misdemeanor such that the president’s removal from office is constitutionally justified. They are, in short, irrelevant witnesses.

But if a Biden is a witness, his actual testimony would not matter because Mr. Trump, loudly echoed by his political and media allies, would endlessly insist that the Biden family’s impropriety had been proven beyond any doubt by needing to testify before the Senate about its purported misdeeds. Fox News would roll endless clips of the questions Senate Republicans would ask of either Biden while keeping the cameras on the witness, then cutting away before showing the answers. This would be enough — more than enough — to lead many Americans to believe that Joe Biden indeed had done something wrong, even if they couldn’t say exactly what it was.

That would be a dangerous infection of America’s 2020 election and an even more damaging step for American democracy over the long haul. In 2016, election interference began with Russian disinformation and continued with America’s amplifying it. In 2020, we’re already seeing election interference that begins with American disinformation and then continues with Russian social media accounts amplifying it. Testimony from either Biden would bring that disturbing trend onto the hallowed floor of the Senate.