The falling prices of cable-TV stocks and the ghosts of failed megamergers past are casting a pall over Comcast Corp.'s proposal to buy AT&T Corp.'s cable division.

Comcast, which was trading above $38 a share when it agreed Dec. 19 to buy AT&T Broadband, closed at $25.61 Friday.

In practical terms, that means the deal has lost some of its sweetness for AT&T shareholders, who will get about one-third of a share of common stock in the new company, AT&T Comcast, for each AT&T share when the deal closes.

The stock portion of Comcast's stock-and-debt offering was worth $47 billion to AT&T shareholders when the deal was announced. It's worth $31.6 billion today.

Comcast is "as frustrated as anybody" in the industry by the falling stock price, a person close to the company said.

But the company remains dead set on completing the merger and making it work. In the negotiations leading to the deal, Comcast insisted there be no "collar" - that is, no floor price on Comcast's stock at which AT&T could automatically withdraw.

Deals without collars were common in the heady stock market days of the late '90s, but more recently, "given volatility, people have been more focused on collars," said Morton Pierce, head of law firm Dewey Ballantine's mergers-and-acquisitions practice.

Shareholders of AT&T and Comcast are to vote separately on the proposal July 10. They'll also vote on whether to accept the controversial management structure proposed by Comcast, which would make it difficult to remove its president, Brian L. Roberts, from the top spot at the merged AT&T Comcast before 2010.

If shareholders and regulators approve, AT&T Comcast will become the nation's largest cable and high-speed Internet company, with 22 million subscribers.

A big factor in the recent drop of cable stock prices has been the scandal at Adelphia Communications Corp., which began to unravel after disclosing in April that it had guaranteed billions of dollars in off-the-books loans to the founding Rigas family.

Stock analysts, however, have rushed to dissociate the practices of Comcast and others from Adelphia.

Niraj Gupta, an influential cable analyst with Salomon Smith Barney, wrote in a recent report that investors' "concerns related to industry accounting practices are overblown," and that cable stocks were taking an undeserved beating.

Analyst Ray Katz of Bear Stearns wrote that "the recent maelstrom of negative news flow out of Adelphia has been unjustifiably weighing down the cable sector. We do not believe the rest of the companies we follow are involved in accounting irregularities and special purpose entity co-borrowing."

Comcast - which is assuming $20 billion in AT&T debt as part of its offer to buy AT&T Broadband and will have a total debt load of $31 billon when the deal closes late this year - has firmly stated it has no off-the-books liabilities similar to Adelphia's.

The company also has vowed to maintain its investment-grade credit rating. It plans to pare debt through the sale of an AT&T partnership in AOL Time Warner Inc.'s Time Warner Entertainment as well as the sale of stock that Comcast currently has in AT&T and Sprint PCS.

Still, that leaves the biggest question of all: Can Comcast deliver on its promise to make AT&T Comcast a profit machine?

The track record for multimedia megamergers isn't good. For instance, AT&T's own attempt to assemble a cable and telecom empire stumbled.

When AT&T began its pursuit of a "broadband millennium" in earnest with the June 1998 purchase of cable company Telecommunications Inc., its stock traded at $34. It peaked near $50 in 1999.

Friday, it closed at $10.18.

And AOL Time Warner so far has also failed to realize the expected benefits of its merger: When No. 1 Internet access service America Online Inc. announced in 1999 that it would buy media conglomerate Time Warner Inc., company officials gushed about the synergies to be reaped in their rich combined strengths in content and distribution.

AOL would provide high-tech expertise and access to millions of online subscribers, it said, while Time Warner would contribute its extensive information and entertainment content as well as high-speed connections through its cable systems.

But AOL Time Warner, now in the midst of a leadership reshuffling, has performed listlessly. AOL shares traded at $72 when the deal was announced in January 2000. On Friday, they closed at $16.40.

"There's a lot of similarity between AOL Time Warner and the Broadband-Comcast thing," said Fariborz Ghadar, a professor with Penn State's Smeal College of Business Administration. Both involve taking large firms and trying to blend different cultures, business processes and technologies.

Ghadar said AOL Time Warner faltered because "AOL doesn't have the bandwidth right now to put what Time Warner offers on their system." The majority of AOL's subscribers have dialup connections, which are too slow to receive rich multimedia content such as video on demand and high-fidelity music.

He said AT&T Comcast can fulfill its promise if it figures out a way to get must-have content flowing over its cable and high-speed Internet network: "The guy that puts the content on the system saves everybody."

Comcast says it does not expect problems with the integration issues that have plagued AOL Time Warner, because the businesses of Comcast and AT&T Broadband already are so similar.

There's even a running joke at Comcast's headquarters about AT&T Broadband: "The last thing we need is a vision."

In other words, save the lofty plans and gee-whiz technology for later.

"What we want to do is talk about nuts and bolts" of running cable systems, the person close to Comcast said.

Evan Rothschild, an equity analyst at mutual-fund company Delaware Investments in Center City, put Comcast's sagging share price in this perspective: "Would I prefer that Comcast was still trading at $40? Sure... but if you look at what all of telecom has done, it has not been a good year.

"As an AT&T shareholder, I'd say we're still very positive on what the deal can mean."

Still, there are doubters who think AT&T Comcast's debt load could overwhelm the company.

"The problems we've identified are so systemic to the entire structure of the deal, a deity itself could not fix them," said telecommunications lawyer Thomas D. Creighton.

Creighton's law firm advised municipal cable commissions in the St. Paul, Minn., area to reject the transfer of local cable-franchise licenses to AT&T Comcast because the merged company's finances would be too shaky.

The heavy debt means the company would have to raise customers' rates, slash customer-service jobs and skimp on facilities and equipment.

He said the companies initially refused to share data that might offer a financial rationale for the merger. After his firm gave its negative recommendation to the cable commissions, the companies asked for time to provide more information.

"Don't just tell us you're good at it, and you're Brian Roberts, and 'trust us,' " Creighton said. "Show us."