As drive-time listeners of KSCJ talk radio in Sioux City navigate the snow-swept plains of western Iowa, a familiar voice keeps piping up in between the weather updates and Rush Limbaugh diatribes: that of Sam Clovis, a local professor and onetime radio host himself, urging listeners to support Donald J. Trump.

“I trust him, and that’s the highest praise I can offer,” he says, over swelling piano and violins.

Television viewers can tune out, and online audiences can scroll by or click “Skip this ad.” But radio listeners, stuck in their cars for long stretches, may be the closest thing to a captive audience for political commercials. And in Iowa and New Hampshire, at least, they are already being pummeled with appeals, both positive and negative, by the presidential contenders and their allies.

A small group backing Senator Ted Cruz of Texas is bashing Senator Marco Rubio of Florida with radio ads mocking him as nothing more than a pretty face. An ethanol industry group, meanwhile, is running radio ads in Iowa calling Mr. Cruz “the worst kind” of politician over his opposition to ethanol subsidies popular in the state.

The sprawling Republican primary field naturally has a lot to do with it, but campaign commercials on radio appear to be having a moment: Political advertising on iHeartMedia, one of the biggest radio conglomerates in the country, with 858 stations in more than 150 markets, was already up 30 percent for the fourth quarter over the same period in 2011, the company said.