Vice President Biden’s vow that he’ll refuse to comply with a subpoena to testify at the impeachment trial of President Trump certainly puts the hay down where us mules can get to it. The fact is that there’s not one particle — not a gluon — more of principle on the Democratic side of this proceeding than the Democrats ascribe to the Republicans. This is a political fight from top to bottom, left, right, and center.

That’s what we take from the interview Mr. Biden gave on Friday to the editorial board of the Register newspaper in Des Moines. “Testifying before the Senate,” the paper paraphrased the ex-veep as saying, “would take attention away from Trump and the allegations against him.” Never mind that the charge of abuse of power that the Democrats have laid on Mr. Trump is his effort to force an investigation of Mr. Biden.

The ex-veep went so far as to suggest that one reason he wouldn’t testify is because he didn’t trust the press. “What are you going to cover?” Mr. Biden demanded of the executive editor of the Register, Carol Hunter, who had put the question to him. “You guys are going to cover for three weeks anything that I said. And (Trump’s) going to get away. You guys buy into it all the time. Not a joke.”

Mr. Biden dug himself in even deeper on Saturday. He sent out a tweet to “clarify” what he told the Register. “In my 40 years in public life, I have always complied with a lawful order,” he asserted, “and in my eight years as VP, my office — unlike Donald Trump and Mike Pence — cooperated with legitimate congressional oversight requests.” The question is whether it is the witness or the Senate who decides what’s legitimate.

Mr. Biden’s intransigence on this head is all the more remarkable in that he served for eight years as president of the Senate, which would be issuing any subpoena for testimony in the impeachment trial. So one would think that Mr. Biden, above all parties in this proceeding, would favor deference to the independent powers of the upper chamber. Instead, he is asserting that he will defy the institution he served for so long.

How is this any more acceptable to the Democrats than is Mr. Trump’s behavior, about which Speaker Pelosi and Senator Schumer are carrying on? Our own view is that Mr. Biden’s refusal to cooperate is less acceptable. Mr. Trump has at least real standing to make a claim in respect of separated powers. He’s in office as the head of the only branch where the constitutional grant of power is made to one individual.

Even were Mr. Biden still vice president, it’s hard to see how he might have more of a claim to defy the Senate than he does now. In any event, he’s no longer in office and neither is — nor ever was — his son, Hunter. So it’s hard to see how either Biden could escape being called by a Senate that wanted to hear from them. Imagine a witness in an Article III trial refusing to testify on the grounds that it would be a distraction.

The view of the Sun is that what is constitutional for the goose is constitutional for the gander. If anyone is to be called, then both sides logically get to call whom they want. Better yet, if the House is going to stand on its constitutional prerogative and determine its own rules, which it did in impeaching Mr. Trump, let the Senate, and it alone, determine its own rules. If Mr. Biden refuses to appear, let him answer for contempt of the institution over which he once presided.