House Minority Leader Kevin McCarthy (R-Calif.) is a key witness to the Jan. 6 Capitol attack. So it makes sense that the House committee investigating the riot would want to interview him. But Mr. McCarthy declared on Wednesday that he will not cooperate, calling the investigation an illegitimate abuse of power designed to damage Democrats’ political opponents.

This is the man who boasted in 2015 that House Republicans had employed their Benghazi investigation to wound Democratic presidential hopeful Hillary Clinton — who, by the way, testified voluntarily for 11 hours before the GOP’s show trial, which produced no evidence of wrongdoing by the then-secretary of state in connection with the 2012 terrorist attacks in Libya that killed four Americans.

The Jan. 6 probe, by contrast, is of vastly more consequence. It represents the country’s best chance to compile an authoritative account of how and why a sitting president tried to overturn a free and fair election and, when he failed, spurred a mob to attack Congress as it counted electoral college votes. This review is indispensable not only for history but also to bolster the nation’s democratic procedures against another attempt to subvert them, which could come as early as 2024.

No patriotic American should oppose such a probe. Yet, from the beginning, the minority leader has sought to impede any reasonable inquiry. First, he pushed to scuttle a bipartisan Jan. 6 commission. After Republicans killed that proposal, Democrats empaneled a special committee of House lawmakers. Mr. McCarthy tried to pack it with pro-Trump sycophants who would have obstructed its work. He now argues that, because Democrats balked at his efforts to sabotage the committee, it is illegitimate.

Mr. McCarthy likely has direct knowledge of then-president Donald Trump’s state of mind on and around Jan. 6, as well as details of Mr. Trump’s weeks-long effort before the attack to overturn the 2020 election results. Indeed, the minority leader spoke with Mr. Trump during the riot, and reports suggest that Mr. Trump ignored Mr. McCarthy’s pleas for help. “Well, Kevin, I guess these people are more upset about the election than you are,” he reportedly said.

It is doubtful that Mr. McCarthy’s testimony would be flattering to Mr. Trump, which might jeopardize the minority leader’s chances of becoming speaker, should Republicans regain their House majority this fall. But subpoenaing Mr. McCarthy is more than justified; indeed, the minority leader is such an important witness, it would be a poor investigation if the committee failed to compel his testimony. “He’s clearly trying to cover up what happened,” committee vice chair Liz Cheney (R-Wyo.) said Wednesday. “He has an obligation to come forward, and we’ll get to the truth.”

Subpoenaing the minority leader would be unprecedented, but his behavior amounts to a dereliction of his oath to support and defend the Constitution against all enemies, foreign and domestic. In his quest to become the next GOP House speaker, Mr. McCarthy has instead thrown in his lot with the enemies of democracy.