It's bad enough that Research in Motion Ltd. is trying to fend off patent-infringement claims that threaten its popular BlackBerry. The Canadian company faces an even graver challenge because competitors are taking aim with new products that could challenge BlackBerry's long-held dominance as the handheld e-mail device of choice.

"I think RIM is facing the most critical time in its corporate history for those two reasons," said John Jackson, an analyst with the Yankee Group. "Are they vulnerable? Yes."

It could be RIM's success that is paving the way for its decline. BlackBerry pioneered wireless e-mail, making it so popular that "BlackBerrying" became a verb, like "Xeroxing." RIM cornered the market by selling BlackBerry software and the hardware -- complete with the miniature keyboard -- that pushed e-mail to anywhere in reach of a cell phone signal.

But popularity breeds competition. Although early BlackBerry users included a relatively small number of executives, wireless e-mail now appeals to a wide consumer market that is irresistible to competitors. As the market heats up, sales of the BlackBerry are beginning to cool.

If RIM loses its footing, analysts say, it could relegate BlackBerry to the minor leagues of technology history -- like Atari video games or Sony Corp.'s Betamax video recorder.

James L. Balsillie, RIM's co-chief executive, said the troubles would not be long term for RIM. The company will shoot down both the patent case and threats from competitors, he said.

"This business is going really, really well," Balsillie said. By next year BlackBerry will expand globally and come out with more devices that appeal to different kinds of business users, he said. "Competition has been constant over the past 10 years," he said, pointing out that Microsoft Corp. has tried without success to develop a product to challenge BlackBerry.

But first, RIM must resolve a years-long dispute with Arlington-based NTP Inc., which has threatened to shut RIM down for violating some of its patents. Last week, a federal appeals court denied RIM's request to reconsider an earlier decision that found RIM in violation of several of those patents.

RIM said it would appeal its case to the Supreme Court and challenge the validity of NTP's patents, which were rejected in an initial review by the U.S. Patent and Trademark Office.

It will not threaten the business, Balsillie said. "If the worst comes to worst comes to worst, we have a work-around anyway" -- a kind of technology tweak that would allow BlackBerry to maintain service, he said.

Meanwhile, RIM's sales growth is slowing. Last quarter, its net subscriber growth declined 8 percent compared with the previous quarter -- even as the overall popularity of computer-like smart phones increased.

That erosion could accelerate. Even though RIM has the biggest market share, with 3.65 million subscribers, dozens of software and device makers such as Microsoft, Palm Inc., Good Technology Inc. and Nokia Corp. are partnering with each other and developing new devices and software programs that are aiming to take a huge bite out of the BlackBerry.

RIM is trying to adapt by forging similar partnerships and licensing its software to other manufacturers.

But RIM is at a disadvantage because its position as the market leader makes it the target of everyone else, said Roger Entner, an analyst with Ovum. After several years of trying to catch RIM, rivals are finally figuring out what works, he said.

Terry Austin, president of worldwide sales for Good Technology, said his software company is siphoning off market share from BlackBerry, particularly as businesses expand the use of the technology among employees. "We have half of the Fortune 50 customers," he said. "A lot of them had BlackBerrys, but they've introduced us."

Entner said the real threat to RIM is Microsoft and its established dominance in the workplace. Microsoft has a long history of using its clout to unseat smaller rivals, he said. "I think RIM has a similar future as a lot of other once-dominant companies that Microsoft eyed," he said. "You might survive, but you're diminished."

Other firms, including Microsoft and Good Technology, are trying to capture some of Research in Motion's market.