Despite knowing full well who David Duke is and what the Ku Klux Klan is, Donald Trump feigned ignorance of both in a national television interview two days before an election in southern states. Duke, a former grand wizard of that terrorist and racist organization, had endorsed Trump's candidacy. CNN's Jake Tapper was throwing Trump a softball by giving him a chance to disavow the endorsement.

Instead, Trump pretended he'd never heard of Duke or the KKK, about whom he wrote in his 2000 book and even commented on briefly last week, and protested that he could not condemn them without doing some research. (You can watch the video above.) The look of disbelief on Tapper's face when his guest wouldn't even take that step toward the bounds of decency is something to behold.

With this ruse, Trump may have avoided alienating the white supremacist and white nationalist supporters whom his candidacy has emboldened. Perhaps he thinks their support is needed to carry southern states on Tuesday. But such intellectually dishonest gestures as this should alienate all people of good will, whether they are down-the-line conservatives or those of much more mixed opinions who have gathered around Trump. Although this is not the first demonstration that Trump is unsuited to be president, it is the clearest yet.

Over the course of the primary campaign, Trump has become increasingly insouciant toward basic decencies of political discussion. To be sure, not every taboo Trump has broken over the course of the last six months deserved to remain intact. Part of his appeal to the average conservative is that most of them share a healthy disdain for political correctness.

But as he has broken through nearly all of them with impunity, he has accumulated contempt for almost anything that decent people hold dear. He appears to be seeing how far he can go, and it is taking him further and further into the fever swamps. There, he seems incapable of discerning the difference between left-liberal humbug that deserves to be smashed, and the propreties of a modern democracy in which all people are equal in law and in dignity as human beings.

At this point, thanks to this demonstrated poor judgment, a Trump nomination will probably guarantee the election of President Hillary Clinton.

Republicans and all people who want sober and humane conservative government in America, who want those qualities and policies embodied in the person residing at the White House and in the people holding the majority in the House and Senate, must say no to Trump on Tuesday.