“...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.” (NYT)

Legitimacy is the test for any government, including democratically elected governments. A democracy is process-centered, not result-centered.  In challenging the legitimacy of the impeachment process, and the Constitution with it, and as he did the 2016 election Trump is challenging the legitimacy of democracy itself. He is centered on result alone. If he wins, the process is legitimate but if he loses, it’s all illegitimate.