The problem is that an ad accusing Trump of not being a conservative will appeal almost exclusively to GOP voters who are strongly conservative. But those voters are mostly already supporting other candidates. Trump’s base of support lies elsewhere, and might end up largely unaffected by a he’s-not-one-of-us ad campaign.

The short version of the problem could be this: An attack ad says Trump is not a conservative. Trump supporters — and other possible GOP voters, as well — say, that’s OK, we’re not conservatives, either.

A just-released Wall Street Journal-NBC poll has some evidence to support that possibility. The poll divided the Republican electorate into those who described themselves as very conservative, those who described themselves as somewhat conservative, and those who described themselves as moderate, or even liberal. (That last group makes up about 30 percent of the GOP electorate.) Trump has significantly higher support among moderate Republicans than among conservatives; he is the candidate of 31 percent of moderates, versus only 20 percent of those who call themselves very conservative. (Ben Carson, in contrast, is the choice of just 14 percent of GOP moderates, and 28 percent of those who call themselves very conservative.)