EVERY New Year’s, in a spirit of self-examination, I try to catalog my worst blunders from the preceding year. But this year, like almost every pundit in America, I have one mistake that overshadows all the others, one confession that makes my other faults seem venial by comparison.

I underestimated Donald Trump.

To really make a clean breast on this issue, I have to reach back earlier than 2015 (some forecasts take more than a year to be disproven), to a column I wrote in the far-off days of the 2012 campaign, when Mitt Romney flew to Vegas, baby, to accept an endorsement from the Donald.

This struck me, at the time, as a needless move by Mitt, because it left him sticky with the tar of Trump’s birther nonsense while delivering little in return. The idea that Romney needed the kind of voters excited by Trump’s flamethrower style, I wrote, confused “the existence of a fan base (which Trump certainly has) with the existence of a meaningful constituency (which he almost certainly does not).” And even if there were real Trumpistas, Romney would win their allegiance eventually: “Anyone who thrills to Trump’s slashing attacks on the president probably isn’t sitting this election out.”

As a third-party candidate, I went on, Trump might pose some danger to Romney’s general-election chances. But Trump’s “third party rumblings are like his birther bluster — sound and fury, signifying only ego.” And Romney would risk little with conservatives by giving him the stiff arm. “Trump isn’t Rush Limbaugh or Sarah Palin: His conservatism is feigned, his right-wing fans are temporary admirers with no deep commitment to his brand or cause, and hardly anyone in the conservative media is likely to rise to his defense.”