Trouble at two key business partners, product delays, and the impending arrival of Howard Stern on the airwaves of a competitor have combined to drive the stock of District-based XM Satellite Radio Holdings Inc. down more than 20 percent over the past four weeks.

Stern's January start at Sirius Satellite Radio is expected to drive more than a million subscribers to the New York-based service, potentially cutting into XM's lead in customers, analysts said.

Though XM has 5 million subscribers, more than twice the 2.2 million of its competitor, Sirius reported faster subscription growth over the past three months and predicted a strong period ahead as Stern fans sign up.

Although XM had positive news last week when it released its most recent earnings report -- a 48 percent increase in new subscribers and a more than doubling of revenue from the year before -- the stock price still dropped 10 percent that day alone. XM closed yesterday at $27.85.

The issues run beyond the impact of Stern. The bankruptcy filing of Delphi Corp., one of XM's chief hardware makers, has raised concerns about the possibility of supply disruptions stemming from a labor dispute. The number of cars in which General Motors Corp. said it plans to install XM in next year fell below some investors' expectations during a downturn in the auto market. A highly anticipated device from Samsung that combines satellite radio with an MP3 player, meanwhile, has been delayed until after Christmas.

"Management's still got explaining to do," read the title of an Oct. 28 research note by CIBC World Markets Inc. analyst Jason Helfstein.

Helfstein and other analysts noted, however, that none of these issues is a long-term problem. "We believe all these issues can be resolved in XM's favor," Helfstein said in his note.

Hoefer & Arnett Inc. analyst April Horace wrote in a recent note that the market's reaction to such concerns was "overdone."

"The fundamental business model is intact," Horace said in an interview. "The path for growth continues to be strong."

XM charges $12.95 a month for 150 channels of music, news, talk, entertainment and sports on special XM satellite radio receivers. Sirius charges the same for about 120 channels.

Neither company has made a profit so far. Sirius yesterday reported that its net loss widened to $180.5 million, or 14 cents per share, from $169.4 million, also 14 cents per share, the year before. Revenue more than tripled, to $66.8 million from $19.1 million.

XM officials said they don't anticipate any fallout from Delphi's troubles, which involve unionized workers in the United States. Delphi makes XM hardware overseas with nonunion labor.

XM officials and several analysts pointed out that XM still dominates the auto market and has deals with 60 to 70 percent of U.S. automakers. Honda, Hyundai and Toyota also plan to increase the number of cars that come with XM as an option, which would make up for any slowdown by GM. XM gets about half its subscribers from the auto market and the other half through retailers such as Best Buy and Circuit City.

The company's biggest challenge in the coming months may be on the retail front, as Sirius begins an ad blitz promoting Stern.

"Stern may be the one risk to watch as the retail market share temporarily shifts to [Sirius]," wrote Tom Watts of SG Cowen & Co. in an Oct. 26 note on XM.

Analysts from companies such as J.P. Morgan expected Stern to add 1 million to 1.4 million subscribers to Sirius' subscriber rolls in the next year -- revenue that would help offset the company's five-year, $500 million deal with the radio host.

During an analyst call yesterday, Sirius chief executive Mel Karmazin predicted that Stern, lower prices on hardware and a new portable device would produce a "blowout" fourth quarter.

"Could [XM] get eclipsed by retail ads from Sirius for two quarters because of Stern? Sure," Horace said. But in the long term, she added, XM still has an advantage because of its lead in auto distribution.

XM officials have their own holiday marketing blitz planned. And it doesn't involve taking Stern on directly.

"We're focused on what we offer. We have our own unique content. We don't want to be Stern satellite radio," said spokesman Chance Patterson. "We have our share of big names . . . but focusing on big names doesn't address the user experience. People want a lot of live content and a mix of sports and news. We've been able to deliver the mix people want."

NASCAR driver Jimmie Johnson is joining XM Satellite Radio as a host. The service has 5 million subscribers.