Yet many of Mr. Trump’s positions have an improvisational air, shifting in their specifics as he seems to dream them up or reconsider them on the fly and out loud, in free-associative speeches or shoot-from-the-hip interviews.

What some have called “Trumpism” is founded not on a specific agenda, like the bullet-point Contract With America in 1994 that led to the Republican takeover of the House. Rather, it is built on boiling grass-roots anger over the ineffectiveness and scripted talking points of conventional politicians on matters like illegal immigration and America’s global power.

“Everybody in the establishment misunderstands the game he’s playing,” said Newt Gingrich, the author of the Contract With America and onetime House speaker who was himself a Republican presidential candidate in 2012. “His opponents want to talk about policies. He’s saying if you don’t have a leader capable of cutting through the baloney, all this policy stuff is an excuse for inaction.”

Waffling, flip-flopping and inconsistencies, all of which might hobble a conventional candidate, have not dimmed Mr. Trump’s appeal to his Republican supporters.