Rush Limbaugh, the most successful talk-radio host in history, is ailing. And so is the medium he helped revolutionize over the past 30 years.

Faced with aging and shrinking audiences, competition from newer technologies and financial problems for the biggest station owners, talk radio is in decline — both as a business and a political force. Once a leading platform for popularizing conservative candidates and policies, talk radio is on the verge of becoming background noise, drowned out by a cacophony of voices on podcasts, cable TV and social media.

The format’s crisis comes as its biggest star is battling to stay on the air — indeed, he is battling for his life. Limbaugh, 70, has been frank about his struggle with what he said last year is advanced lung cancer. “I wasn’t expected to make it to October, and then to November, and then to December,” he said on the air just before Christmas. “And yet, here I am.”

Limbaugh’s uncertain future confronts the talk-radio business, and conservatism generally, with the prospect of losing its most galvanizing figure. Since leaping from a local station in Sacramento to nationally syndicated stardom in 1988, Limbaugh has been the bullhorn behind every important conservative initiative, from the Contract With America in the mid-1990s, to the tea party movement of the Obama era to the ascent of Donald Trump.

“He will leave a huge void when he leaves,” Paul D. Colford, a Limbaugh biographer, said. “There is no one who has come up to replace him. There is no new voice out there. There is no one like him.”

From his earliest days on the air, Limbaugh trafficked in conspiracy theories, divisiveness, even viciousness (“feminazis” was one of his infamous coinages). He created what Columbia University historian Nicole Hemmer calls a kind of “political entertainment” that partially supplanted traditional conservatism and was crucial to Trump’s political ascendancy.

Early in the coronavirus pandemic, Limbaugh told listeners that the virus was no worse than “the common cold” and that the news media had “weaponized” the crisis to hurt Trump. He floated the fringe theory that the virus was created in a Chinese laboratory as a bioweapon. A few weeks before the November election, he devoted two hours of his program to a worshipful Trump interview. After Trump lost, Limbaugh amplified the president’s lies about voter fraud and at one point suggested that conservative states might secede from the union.

Trump, for his part, awarded Limbaugh the Presidential Medal of Freedom during the State of the Union speech last year.

“Limbaugh [wasn’t] just as an instrument of Trumpism but a precursor to it, part of the transformation of the Republican Party into a party captive to its base and reliant on right-wing media,” said Hemmer, the author of “Messengers of the Right: Conservative Media and the Transformation of American Politics.”

That raises the question of what will be left of that legacy after the conservative radio ecosystem he created disintegrates.

Gauging Limbaugh’s audience has always been a matter of guesswork; the ratings tracker Nielsen and its forerunner, Arbitron, have never done a national audit of his listeners.

Limbaugh maintained for years that he attracts some 20 million listeners a week, a figure still cited in media accounts more than 20 years after he first asserted it. More recently, he has claimed he reaches 43 million people a week.

The figures are undoubtedly subject to a little Limbaugh-ian grandiosity. An industry trade magazine, Talkers, thinks the real number is somewhere around 15 million listeners. “It goes up and down with minor fluctuations,” Michael Harrison, the magazine’s publisher, said. “Rush has held steady. His audience is mega-loyal.”

But conservative talk radio’s foremost problem isn’t so much how many people are listening as who.

The audience that grew up with Limbaugh is now quite gray, largely people 65 and older. Fewer than 8 percent of those who regularly listen to talk radio (including public radio) are 25 to 54, according Nielsen’s research.

Meanwhile, plain old AM-FM radio — the primary medium for talk programs — is rapidly losing ground to newer technologies such as satellite radio, streaming audio and podcasts. Only 50 percent of those surveyed by Edison Research last year listed terrestrial radio stations as their first listening choice in a car.

The shift makes someone like comedian Joe Rogan, a libertarian with a hugely popular podcast, “the next Rush Limbaugh,” says Paul Matzko, the author of “The Radio Right: How a Band of Broadcasters Took On the Federal Government and Built the Modern Conservative Movement.”

Conventional radio’s decline appears to have accelerated sharply during the pandemic. Daily commutes have been disrupted by stay-at-home orders and a new work-from-home culture, wiping out part of radio’s lucrative “drive-time” audience. It’s unclear whether those listeners will return after the pandemic subsides and traffic jams return.

Talk radio’s older demographics were a weakness even before the pandemic, said Jerry Del Colliano, a New York University business professor who publishes the Inside Music Media blog. Turned off by the hosts’ occasional nastiness and seeking young customers, many blue-chip companies have stopped advertising on talk-radio shows, leaving the stations with smaller sponsors who pay less, he said.

Limbaugh-style talk radio was originally a solution to the industry’s technical and business problems. By the 1980s, AM radio had lost out to the higher-fidelity FM band as the preferred medium for music, the most popular kind of radio programming. As rock, pop, country and other formats migrated to FM, AM stations faced an existential crisis.

What to do? The answer: talk and news, neither of which required high fidelity.

Limbaugh proved to be not only wildly popular but also cost-effective. In 1988, a savvy former ABC Radio executive named Ed McLaughlin signed the host — then working at a little-known Sacramento station — to a nationwide syndication contract. McLaughlin offered Limbaugh to stations at an unbeatable price: free. All they had to do to carry his program was to set aside four minutes per hour for ads that McLaughlin’s company sold to national sponsors. The stations got to sell the remaining commercial time to local advertisers.

The formula proved so popular that stations signed up en masse. Within a few years, Limbaugh was carried on more than 600 stations. His brash, outrageous and often comical style began to revolutionize American political culture as an army of “Dittoheads” flocked to “Rush Rooms” set up in restaurants nationwide to eat lunch and hear their hero.

Many of the hosts who followed Limbaugh into national syndication weren’t as skillful or as popular. But with an assist from the Reagan administration — in 1987, the Federal Communications Commission repealed the Fairness Doctrine, which had required stations to present politically “balanced” programming — the new wave of “hot” talkers enabled stations to create entire schedules consisting of nothing but conservative chatter. To this day, almost all of the most popular talk hosts are conservative or libertarian.

Much has changed since then. According to Nielsen Research, news-talk is still the most popular of the many formats on the radio. During an average 15-minute segment in 2019, 9.5 percent of the radio audience was listening to a news-talk station, a slight decline over the previous three years. But the share falls precipitously among younger listeners: Only 6.7 percent of those ages 25 to 54 and 4.3 percent of those ages 18 to 34 listen to talk stations.

The economic, demographic and technological forces now converging on conventional radio helped push the two biggest station owners, iHeart Media and Cumulus Media, to file for bankruptcy-court protection in 2017 and 2018, respectively. The two companies had borrowed heavily to gobble up radio stations. They now own more than 1,200 between them.

The financial pressures on Cumulus and iHeart make them unlikely to invest in the one thing that might save political talk radio: local personalities, according to Brian Rosenwald, a University of Pennsylvania historian who wrote a book called “Talk Radio’s America: How an Industry Took Over a Political Party that Took Over the United States.

He noted that sports-talk radio has fared relatively well by featuring hometown personalities yakking about local teams. But hiring local hosts is an expensive proposition. Rosenwald said AM radio is “in imminent danger, unless someone comes along to invest enough to enable it to become a local medium again.”

All of this doesn’t mean the era of hyperbolic, confrontational conservative talk exemplified by Limbaugh is coming to an end. The medium might be in trouble, but the conservative message is already moving to new delivery systems.

“Right-wing media is still a massive growth industry,” says Hemmer. “When Limbaugh’s show goes dark, it will be the end of an era. But it’s hard to imagine that too much will change: It will take a while for [new] outlets to gain the type of trust that Limbaugh has . . . but all in all, we’re living in a political culture Limbaugh helped create, and it’s likely it will continue to exist long after his show ends.”