Politics on the radio is taking an enormous leap: from the era of the occasional fireside chats to an around-the-clock bonfire for the coming presidential race. But there are plenty of reasons to be skeptical of the claim that constant radio coverage of the campaign that will last for the next 500 days, in a country of 300 million, will actually find a far smaller audience than Franklin D. Roosevelt’s discourses on the New Deal and World War II, which aired just 30 times over 11 years and in a country less than half as big as it is today.

This week, XM Satellite Radio is offering its listeners a four-day preview of one of the newest channels on its expansive digital dial. Called “POTUS ’08” — after the acronym the Secret Service uses for the “president of the United States” — Channel 130 is going to be the first radio station dedicated to all-presidential-election news, all the time. The actual launch is planned for September, but XM made plans for live coverage of yesterday’s CNN debate by the Democratic candidates, to be followed today by replays, some original programming and highlights of campaign speeches. Tuesday afternoon and evening, the focus will be on the CNN Republican debate. After another 24 hours of replays, the test will end Wednesday evening.

It sounds great for political junkies, a sort of C-SPAN radio on steroids, available coast to coast. And just like YouTube.com, it could lessen the impact of the mainstream media on the political process, giving lesser-known candidates another avenue into people’s homes — and, in this case, into their cars as well.

But in the age of TiVo, on-demand programming, the Internet and 500 or more channels of cable television in many markets, can a satellite radio station hope to find a critical mass for a mixture of campaign tidbits, call-in programming, polling reports, debates and all manner of campaign analysis? Could this newest niche for political speech offer an important platform for one side or the other — as television did for John F. Kennedy in 1960, conservative talk radio shows did for Republican congressional candidates in 1994 and the blogs did for Democrats last fall?

“It’s an old medium that can be used in a new way,” Larry J. Sabato, director of the University of Virginia’s Center for Politics, said of radio, because it is still the best way to reach millions of urban and suburban voters stuck in commuter traffic every day. “It’s a niche market, not something that will dominate political discourse for millions,” Sabato said. “But elections are sometimes decided by niches.”

Radio was an important medium in politics even before FDR. Ironically, Calvin Coolidge — so reserved he was known as “Silent Cal” — was the first president to have his stump speeches, addresses to Congress and an inauguration put on the air. In the 1920s, families gathered around the radio in the evenings much as they did around the TV starting in the 1950s. But listening and viewing habits have changed, of course, making the sort of round-the-clock availability that XM promises increasingly important today.

XM says POTUS ’08 is being launched as a public service that might increase interest in the medium — which is why it will be available commercial-free to anyone with an XM radio, even those who don’t buy the 170-channel service. “We’re hoping that candidates see this as a way to talk to satellite-radio listeners,” said Kevin Straley, the senior vice president of news, talk and sports programming.

Straley says XM is in the market for nonpartisan and objective journalists, as well as talk show hosts and producers, to staff the channel. The content mix is still under development, he said, but he envisions drive-time call-in shows, original coverage from the campaign trail, recordings of current candidates’ speeches, stories about past campaigns — and, of course, interviews with candidates. The channel has announced a partnership with C-SPAN and may also feature feeds from XM’s slew of other news partners, including Fox, CNN, ABC and the BBC.

XM, headquartered in Washington, topped 8 million subscribers in April. Its proposal in February to merge with the nation’s only other satellite radio network, Sirius, is under review by the Federal Communications Commission and the Justice Department. XM officials deny that their around-the-clock foray into politics is designed to curry favor with policy makers.

Some political bloggers remain dubious about the potential impact. “What C-SPAN and XM are doing is good for public discourse, even if only point-oh-oh-oh-oh whatever percent of America will be listening,” wrote Matthew Felling, co-editor of CBSNews.com’s Public Eye Web site.

In other words, matching FDR’s numbers during the Golden Age of radio may not be the point at all. Instead, the better use for satellite networks — which can create new radio channels easily, without having to go through the arduous licensing process of terrestrial radio — could be to afford every presidential aspirant a means to talk to voters, even in the middle of the night. For Mike Gravel, tune to Channel 198. For Duncan Hunter, try Channel 202.

Contributing editor Elizabeth Wasserman is a Washington freelance writer. She can be reached at ewasserman@cq.com.