Well, America — or at least those 11 million or so voters who cast a plurality of ballots in the Republican presidential primaries and caucuses — you asked for him, and you've got him: Donald Trump is now the party's presumptive nominee. The departures of Sen. Ted Cruz and Gov. John Kasich from the political battlefield after their lopsided Indiana losses on Tuesday mean any reasonable hope of a contested Republican National Convention has officially left the building.

What seemed unimaginable last June, then hard to believe, later possible and eventually probable is now reality and not merely a made-for-television spectacle. Let this sink in for a bit: Donald Trump, pitchman, huckster, insufferable narcissist and celebrity billionaire now stands where Abraham Lincoln once stood — wearing the mantle of the Grand Old Party.

The Republican electorate, allegedly unhappy that past leaders were “not conservative enough” or compromised with Democrats too often or were “political insiders,” have given the nod to a man famous for making deals, for attaching his name to the work of others, for spending millions backing Democratic candidates and for being generally cozy with the Washington establishment. Makes perfect sense, right?

Four years ago, a top aide to Mitt Romney famously spoke of the “Etch-A-Sketch moment” when the Republican nominee would erase what he said during the primary and completely rewrite his image for the general electorate. As Mr. Trump might say, he'd better be packing a huuuge Etch-A-Sketch this time around.

It isn't just that the New York vulgarian has been coarse and insulting (“Little Marco” and “Lyin' Ted”) or that his remarks have often been bigoted and sexist (describing Mexicans as criminals and rapists, proposing to ban Muslims from entering the United States or condemning Hillary Clinton for not being able to “satisfy” her husband), or that he's incited violence at his rallies, it's that he's demonstrated over and over again a vapor-thin and often naive grasp of policy, particularly in the field of foreign relations — from the notion that he could bully Saudi Arabia into financing U.S. military costs to advocating a wall along the southern border that Mexico would pay for to allowing more countries to acquire nuclear weapons.

Just this past week came a classic example of the Trump scorched-earth, brain-dead approach when he linked Senator Cruz's father, Rafael Cruz, with the assassination of President John F. Kennedy — based on the flimsiest and most laughable of supermarket tabloid evidence. Now the man who called Mr. Trump a “pathological liar” and utterly “amoral” in response is going to stand on a stage and offer full-throated endorsement?

Say what you will about the Democratic debates, we don't recall any equivalent moment in the Hillary Clinton dust-ups with Sen. Bernie Sanders. That wasn't because they pulled punches about Wall Street donors or the cost of social welfare programs, but because they didn't start acting like this was a primary school playground where the victor is the one who makes the most juvenile taunts.

Mr. Trump can keep shaking that Etch-A-Sketch and pressing that reset button, but the memory of this ridiculous campaign is going to persist. Ms. Clinton has her share of haters and can be a polarizing figure, especially within the Republican Party, but it's hard to see how the voters who twice elected the still-popular Barack Obama to the nation's highest office are going to jump ship and decide Mr. Trump with his boatload of ill-will, particularly toward women and minorities, is a worthy successor. The question is not whether Republicans will be unified but how long before the down-ballot GOP candidates take steps to distance themselves from the man occupying the top of the ticket? Gov. Larry Hogan, one of nine Republican governors who have remained largely silent on the topic of Mr. Trump, surely understands that discomfort better than most.