Today is Super Tuesday, which rates that title due to the high political stakes of presidential primaries and caucuses in a dozen states.

This also is a day when, according to a consensus of pollsters and pundits, Hillary Clinton and Donald Trump will considerably solidify their already-dominating positions in the races for the major parties’ White House nominations.

Coming off Saturday’s landslide victory in the South Carolina primary, Mrs. Clinton appears primed to do quite well again today, especially in the Southern states where her strength with black voters puts Vermont Sen. Bernie Sanders at a decided disadvantage.

But before Republicans designate the brash and often crude Mr. Trump as their nominee, they should review the potential consequences that such a stupefying choice could produce in November — and beyond.

The business tycoon, playing the immigration card to an absurdly hard-line hilt, says he will build a border wall, make Mexico pay for it and round up more than 11 million illegal immigrants for deportation. That bizarre notion, apparently more than any other issue, has helped Mr. Trump win three straight GOP contests, including the Feb. 20 S.C. primary, by impressive margins. When Mexican leaders complained about his offensive immigration pitch, Mr. Trump responded that he would build the wall “10 feet taller.” So there.

He’s even picked up endorsements over the last week from some prominent Republican politicians, including New Jersey Gov. Chris Christie and Alabama Sen. Jeff Sessions.

Yet many true conservatives, including our state’s governor and both of our U.S. senators, remain vigorously opposed to Mr. Trump’s presidential bid.

Gov. Nikki Haley and Sen. Tim Scott have endorsed Florida Sen. Marco Rubio. Sen. Lindsey Graham, who gave up his own presidential bid more than two months ago, was backing Jeb Bush before he, too, dropped out after the S.C. primary.

Gov. Haley, appearing Sunday on ABC’s “This Week,” was asked by host George Stephanopoulos if Sen. Rubio had moved soon enough to “take on” Mr. Trump. She replied with an insight grounded in basic childhood lessons:

“I don’t know if he waited too long, but I’ll tell you Donald Trump is everything we hear and teach our kids not to do in kindergarten. And we have seen this behavior over and over again that’s unacceptable. And I think what we saw from Marco is exactly what we tell our children also, if a bully hits you, you hit back.”

Gov. Haley is right about Mr. Trump’s bullying tactics. And Sen. Scott was correct Sunday on NBC’s “Meet the Press” to deplore Mr. Trump’s failure earlier that day to denounce the Ku Klux Klan. Mr. Trump, when asked by Jake Tapper on CNN’s “State of the Union” about the endorsement of him by former KKK leader David Duke, did not repudiate it.

Sen. Scott issued this statement: “Any candidate who cannot immediately condemn a hate group like the KKK does not represent the Republican Party, and will not unite it. If Donald Trump can’t take a stand against the KKK, we cannot trust him to stand up for America against Putin, Iran or ISIS.”

Mr. Trump later explained that a malfunctioning earphone prevented him from hearing Mr. Tapper’s question about Mr. Duke. And he had, in a tweet, already disavowed Mr. Duke’s support on Friday.

But regardless of whether that excuse is valid, there’s no reason to believe that Mr. Trump is a real conservative who respects the Constitution.

There is, however, ample reason to believe that his spotty business record (four bankruptcies) and reluctance to release his tax returns could present problems that ultimately derail his presidential ambitions, if not during the nomination race then during the general election.

The most persuasive case against a President Trump, though, remains his superficial, erratic and at times downright ugly policy pronouncements.

For a glimpse of the alarming shortcomings of Mr. Trump’s world view, see George Will’s withering column on today’s Commentary page. Mr. Will also fairly points out that GOP elected officials face a defining challenge from Mr. Trump’s troubling rise to party prominence.

So far, Gov. Haley and Sens. Graham and Scott have passed that test by refusing to give in to the bullying blowhard.

And while it might now seem like a long shot, let’s hope many voters in GOP contests start standing up to him, too — and soon.

Otherwise, it’s not just conservative voters who could face a very grim choice in November.