The announcement on Thursday by Hewlett-Packard that it was considering offloading its PC business, even though it is the undisputed worldwide market leader, was a clear sign of the difficulties.

If H.P. goes through with the idea, it would follow I.B.M., an early PC maker, which was one of the first to recognize the long-term problems and, in 2005, sold its business to Lenovo, a Chinese company. Other American makers like Compaq (acquired by H.P.), Gateway and Packard Bell were absorbed by others or just faded away. Depending on how H.P. sheds the unit — it could sell or spin it off as a separate company — only two American PC makers would remain.

One of them, Dell, struggles for every percentage point of market share.

The other, Apple, prospers. And the reason it does highlights the shift from a PC-centric era to one dominated by smartphones and tablets. H.P., Dell and, indeed, every PC maker worldwide, has been unable to make a tablet consumers feel they must have. At the same time H.P. said that it might spin off PCs, it killed off its tablet, the TouchPad, after just a few weeks on store shelves.

Computer makers are expected to ship only about 4 percent more PCs this year than last year, according to IDC, a research firm. Tablets, in contrast, are flying off store shelves. Global sales are expected to more than double this year to 24.1 million, according to Forrester Research. More than two-thirds of those tablets, however, are sold by Apple. Sales of its iPad pulled in $9 billion in just the first half of the year, or 30 percent more than all of Dell’s consumer PC business in the same period. The joke in Silicon Valley is that there is no tablet market, only an iPad market. (That was also true of Apple and the iPod market.)

The other observation that is no joke: Apple is the only maker with strong PC growth. Spending on desktops and laptops grew 16 percent in the latest quarter, while Dell’s consumer product sales increased 1 percent.

“It’s definitely weighing on the computer makers, and it is something that will weigh on them for some time,” said Louis Miscioscia, an analyst with Collins Stewart.

“The tablet effect is real,” said Leo Apotheker, H.P.’s chief executive, in an interview on Thursday, acknowledging that the TouchPad had failed to live up to expectations and that it would have cost too much to compete. “It’s very different from where the business was going 10 years ago,” Mr. Apotheker said.

On Friday, H.P.’s shares fell 20 percent in reaction to his plans.

Michael S. Dell, Dell’s chief executive, took the opportunity to poke fun at the prospect of H.P. unloading its PC unit by saying in a message on Twitter that “they’re calling it a separation, but it feels like a divorce.” Following up with more sarcasm, he said, “If HP spins off its computer business … maybe they will call it Compaq.”

Mr. Dell was clearly enjoying the moment, but his company faces the same market forces as H.P. Its overall PC business has been flat. Recently, Dell has pared back some of its consumer products, including a 5-inch Streak tablet, while keeping a 7-inch tablet. Together, they eked out barely 1 percent of the market, according to ABI Research.

Like H.P., Dell is pushing its enterprise business, which has higher margins. But David Johnson, Dell’s senior vice president of corporate strategy, said his company had no plans to follow in H.P.’s footsteps and split off its PC business. “We have no plans to change our strategy,” he said.

Tablets remain the hope of other PC makers and phone makers. By next year, tablet sales in the United States will outpace those of netbooks, the mini-laptops people use to surf the Web, according to Forrester Research. Netbooks were considered a salvation for the PC industry when they were introduced a few years ago, but they have since fallen out of favor with consumers.

But buyers see little need to buy any tablet other than iPad, even if it is slightly more expensive than some of its rivals, analysts said.

“The performance still isn’t there for a lot of them,” said Richard Doherty, research director for the Envisioneering Group, a market research and consulting firm. “And it’s not just the product, it’s the ecosystem behind it.”

For that matter, selling tablets is no easier for the smartphone makers. Motorola Mobility, which Google said this week that it would buy, got nowhere with its Xoom. Research in Motion entered the tablet market this spring with a long history of building mobile devices. Still, the company has struggled to get consumers to buy its tablet, the PlayBook, which it introduced earlier this year.

RIM says it shipped 500,000 PlayBooks during its last fiscal quarter. Kevin Burden, a vice president at ABI Research, estimated that only 40 to 50 percent of those tablets found buyers.

Shoppers were not charmed by the PlayBook’s inability to directly check corporate e-mail — they have to connect wirelessly to BlackBerry phones — and lack of applications.

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