WASHINGTON — Breathtaking in scope, defiant in tone, the White House’s refusal to cooperate with the House impeachment inquiry amounts to an unabashed challenge to America’s longstanding constitutional order.

In effect, President Trump is making the sweeping assertion that he can ignore Congress as it weighs his fate because he considers the impeachment effort unfair and the Democrats who initiated it biased against him, an argument that channeled his anger even as it failed to pass muster with many scholars on Wednesday.

But the White House case, outlined in an extraordinary letter to Democratic leaders on Tuesday, is more a political argument than a legal one, aimed less at convincing a judge than convincing the public, or at least a portion of it. At its core, it is born out of the cold calculation that Mr. Trump probably cannot stop the Democrat-led House from impeaching him, so the real goal is to delegitimize the process.

Just last week, Mr. Trump acknowledged that Democrats appeared to have enough votes to impeach him in the House and that he was counting on the Republican-controlled Senate to acquit him. By presenting the inquiry as the work of an unholy alliance of deep-state saboteurs and Democratic hatchet men, he hopes to undermine its credibility, forestall Republican defections and energize his voters heading into next year’s re-election campaign.