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 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 Sen. Ted Cruz of Texas is bashing Sen. 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 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.

Trump, whose campaign has dominated television news coverage of the Republican primary, has bought ads only on radio, with six different commercials playing across the country. A big “super PAC” supporting Cruz, Keep the Promise I, has spent $1.3 million on the radio so far and nothing on television.

Other contenders have poured tens of millions of dollars into television commercials, but with seemingly little to show for it. A super PAC supporting Gov. John R. Kasich of Ohio, New Day for America, bought $5.4 million in television time in New Hampshire, but he sagged in the polls soon after.

Yet radio ads may be more effective at getting a candidate’s message through to its intended audience, some campaign strategists said.

“From Dec. 15 through January, if you want to find a basket full of white noise, go to your Iowa TV set,” said Doug Watts, a spokesman for Ben Carson’s campaign, which has been advertising on radio since the summer. “People are spending millions of dollars, and every other spot is going to be a political spot.”

“You’ve got to find a way around that,” Watts said, adding: “Radio works.”

Television advertising is still king, both in terms of total spending and number of times that ads run, of course, and will continue to eat up the bulk of campaign budgets. But if radio spending is still minuscule by comparison, that is partly because it is so inexpensive.

A campaign wanting to reach voters in the Des Moines market with an effective ad purchase would have to spend about $85,000 on television, said Mike Schreurs, the chief executive of Strategic America, a marketing and advertising firm based in Iowa. To reach the voters to the same degree would cost only $48,000 on radio. (Television commercials would reach roughly three times the geographical area, so radio ads would run more frequently.)

Side by side, the contrast is even starker: A 60-second campaign ad during the 6 p.m. newscast of KTIV, the NBC affiliate in Sioux City, in mid-November cost about $900, according to the station’s public file. A minute-long ad at the same time on KSCJ, the conservative talk-radio station there, went for as little as $20, according to the station’s owner.

Production costs are just as lopsided. “You can go in and record a radio spot and have it done in 90 minutes, and the cost is $1,500,” explained Larry McCarthy, the advertising strategist for Right to Rise, a super PAC supporting Jeb Bush. A typical television ad, he said, could cost 10 times as much.

Few would dispute radio’s cost-effectiveness, but it holds particular weight for Republicans, thanks to the huge and devoted followings of conservative talk-radio hosts like Limbaugh, Mark Levin and Laura Ingraham.

“Talk radio provides a captive audience that is less inclined to futz with the dial in between commercials,” explained Kellyanne Conway, a Republican pollster who is running Keep the Promise I, the super PAC backing Cruz. Radio advertising, she added, was “a little more insidious” than television commercials, “and it’s tougher to ignore.”

With that in mind, major radio companies are stepping up their efforts to help politicians reach the voters they are courting — and to capture more of their advertising spending. Nielsen Media and Experian Marketing Services have teamed up in the top 48 radio markets, for example, to help radio companies show advertisers how to target listeners according to party affiliation, likelihood to vote and more.

In Kentucky, Matt Bevin, a Republican who won the governor’s race last month, turned to radio to broaden his support, said Kenny Day, senior vice president of political sales at iHeartRadio. “We can tell Bevin, ‘If you want to reach Blue Dog Democrats, do this station and this time,’” he said.

Republicans are not the only ones pouring money into radio in the presidential primaries. Sen. Bernie Sanders of Vermont has bought heavily on hip-hop and R&B stations in South Carolina, reaching out to black voters with ads highlighting his record on civil rights. And on Tuesday, Hillary Clinton began running a commercial on largely African-American stations in South Carolina — her first radio ad of the campaign — in which she talks about her mother’s difficult childhood.

Tad Devine, a senior adviser to the Sanders campaign, which has bought radio ads in the four early states, promised more to come. He called the medium “a great closing tool” in reminding voters to get to the polls and what to have in mind when they make their decisions.

“It’s very explicit,” he said.

As the Feb. 1 Iowa caucuses draw near, the radio attacks on the Republican side, at least, are quickly growing nastier, while remaining somewhat under the radar of journalists, strategists said.

“Because it’s a nameless, faceless voice often, they tend to be tougher,” said Russ Schriefer, a media consultant for Gov. Chris Christie’s campaign. “Folks fact-check the hell out of TV ads,” he added. “But it’s rare that a radio ad is ever fact-checked. You can be out there saying stuff on radio that you would never say on TV.”

One radio commercial attacking Cruz prompted his campaign to ask Iowa stations to stop running it. The ad said that he favored an end to ethanol subsidies, a policy that would “threaten rural Iowa and thousands of jobs,” but that he supported subsidies for oil because of his own investments in oil companies and because oil interests had given millions of dollars to the super PACs supporting him. Cruz called the ad “blatantly false,” saying he opposed all subsidies, including those to the oil industry.

A small pro-Cruz group, Courageous Conservatives PAC, arguably hit even harder at Rubio, however, with a radio ad attacking him over his support for an immigration overhaul while suggesting that he had little else to show for his years in the Senate.

“Can anyone think of anything Marco Rubio’s ever done?” the ad asks. “Anything at all besides amnesty? Marco Rubio looks good on TV, but that’s about it.”