Henry Olsen suggests that Trump may have more support than his current polling indicates:

And because Trump draws the bulk of his support from less-educated, working- and middle-class voters, he may be positioned to do even better still—for now. Polling data from Europe shows that parties with similar voter profiles to Trump’s consistently do better in both online polls and at the ballot box than in live-interview polling. And currently Trump is far ahead online.

It’s possible that there are people who want to vote for Trump that don’t want to admit as much to a live pollster, but my guess is that there are far fewer of these people than in comparable European cases. For one thing, there is a stronger stigma in many European countries against supporting vocally anti-immigration parties and candidates than there is here. For another, opposition to mass immigration is a much more widely-held position in the U.S. Besides, my impression of typical Trump supporters is that they don’t care who knows that they support him and that they are proud to support him. There may be some that will cast votes for him without boasting about it, but it seems to me that a large part of so-called “Trumpism” is a strong contempt for the prevailing conventions of politics and the people that enforce them. People that can’t stand Trump (and I count myself among them) may be tempted to think his supporters would be embarrassed by the association with him, but that’s exactly what they aren’t. Nonetheless, Olsen’s argument is worth considering, and it should be kept in mind as a corrective to the assumption that Trump’s support will fade over time.