Mr. Limbaugh, who is syndicated by Premiere, continues to command a much larger audience than any other radio host. According to Arbitron’s spring 2008 ratings report, he reached 3.58 million listeners during an average quarter-hour, while the No. 2 host, Mr. Hannity, averaged 1.65 million.

A middle tier of radio hosts helps fill the schedules of AM and FM stations. “Because you don’t have a commodity like music to rely upon, it is all up to the host,” said Carl Anderson, the senior vice president for programming and distribution at ABC Radio Networks. “They are on stage by themselves.”

The talk-radio formula that Mr. Limbaugh pioneered two decades ago remains evident on the air today. Syndicators look for hosts who are entertaining, have a point of view and, as Mr. Anderson put it, show an ability to “connect with an audience.”

Ask different syndicators and you will hear different claims about the “fastest-growing hosts” on talk radio. Westwood One cites its “Dennis Miller Show,” which is syndicated in 200 markets and has a comedic bent. ABC Radio cites the rapid rise of its two-year-old “Mark Levin Show,” which now counts 175 affiliates. The shows are a less expensive alternative to the Limbaughs and Hannitys of the industry.

With more stations converting to news-talk formats  perhaps with the hope that live talk cannot be displaced by an iPod the way music can be  the middle tier is where most of the movement is. The conservative commentator Monica Crowley is entering weekday syndication through the Talk Radio Network. The CNN anchor Lou Dobbs is signing new affiliates for a three-hour afternoon show. And Mr. Scarborough, the host of MSNBC’s “Morning Joe,” is hosting a radio version on WABC in New York. ABC Radio plans to syndicate it next year, beginning in Los Angeles at KABC.

Mr. Scarborough, the former Republican congressman, did not enjoy his brief stint with Westwood One in 2005. But his MSNBC morning program, which replaced the TV simulcast of “Imus in the Morning” when Don Imus was fired for using racially charged language in 2007, resembles a radio show and attracted interest from radio networks earlier in the year. Mr. Scarborough expects his program to provide more political balance than some others.