If asked about his extreme statements, Mr. Trump’s Republican competitors have two options: They can gingerly applaud the feisty casino magnate, hoping to eventually inherit his supporters while trying to avoid responsibility for what he says.

Or they can disagree with Mr. Trump by emphasizing their own views and distance themselves, respectfully, while trying not to use his name. When Mr. Trump is wrong, this is the better course—but be prepared: He responds with insults. He feeds on controversy.

He has called the rest of the candidates in the GOP field “clowns”; tweeted that Rick Perry “needs a new pair of glasses to see the crimes committed by illegal immigrants”; and retweeted a follower who wrote that Jeb Bush “has to like the Mexican Illegals because of his wife.”

When I raised questions about his seriousness, Mr. Trump tweeted that I had “spent $430 million in the last cycle and didn’t win one race.” (In 2010, he donated $50,000 to the Crossroads political-action organizations for which I volunteer.) In the 2014 cycle, Crossroads groups spent $103 million to help win 10 of 12 targeted U.S. Senate seats and 10 of 13 competitive House seats. Not that facts matter much to The Donald.