It was so obvious, I’m embarrassed I missed it.

Like most of the other talking heads on TV, I was haughtily dismissive of Donald Trump’s candidacy. “It’s apparently open mike day in the Republican campaign for president,” I tweeted last June, after Mr. Trump barged into a relatively placid Republican race with a rambling, riotous speech.

Even as he climbed to the top of polls, I confidently predicted that the outrageous Mr. Trump, as transfixing and ubiquitous as he was, was merely a summer fling. He would fade in the fall, when Republican voters got serious about making a long-term commitment.

Seven months later, Mr. Trump has broken just about every rule of conventional campaigning. Short on policy prescriptions and long on provocation, he has serially — and joyfully — insulted Mexicans, women, Muslims, P.O.W.s, people with disabilities and virtually all of his opponents. Yet a week before caucusing begins in Iowa, he still reigns supreme atop the Republican field.

What seemed impossible is now more than plausible: Donald J. Trump, the self-reverential deal maker, could pull off a hostile takeover of the Grand Old Party.