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A year after it was announced, Comcast’s audacious acquisition of Time Warner Cable remains in limbo as Washington regulators scrutinize the deal. No surprise there. After all, the $45 billion merger would consolidate an already-concentrated industry, uniting the two largest cable operators in the United States.

But in recent weeks, the air of inevitability around the deal has dissipated. With the Federal Communications Commission proposing stringent new rules to govern the Internet, analysts have grown more skeptical about the acquisition being approved. Investors began betting against the combination late last month, with shares of both companies falling sharply before recovering last week.

“The prospects for the deal, while they’re still not bad, have continued to go down,” said Kevin Werbach, a former F.C.C. counsel and a professor at the Wharton School of the University of Pennsylvania in Philadelphia.

Advisers to both companies acknowledge that passing regulatory muster is far from certain. Yet David L. Cohen, an executive vice president at Comcast, expressed confidence that the merger would still be approved but acknowledged that the outcome was hard to handicap.

“This is a bit of a black-box process,” he said. “You don’t really know what’s going on under the surface.”

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In Washington, officials at the Justice Department and F.C.C. are poring over data to decide whether to approve the deal, and what if any concessions the companies must make to satisfy antitrust laws. The review continues even as Comcast and Time Warner Cable are completing the minutiae of their planned merger.

Among the issues being examined by regulators are whether a combined Comcast and Time Warner Cable would have too much sway over how traffic moves around the Internet and how much consumers and companies pay for Internet access.

If regulators allow the deal, the company would control an estimated 35 percent of broadband Internet service coverage and just under 30 percent of the country’s pay television subscribers.

Fueling the uncertainty is a series of remarks made by Tom Wheeler, chairman of the F.C.C., before a vote by the commission on Feb. 26. Mr. Wheeler has proposed regulating the Internet like a utility, under Title II of the Communications Act.

If approved, such a move would give the F.C.C. the authority to ensure that content is not blocked on the Internet, and that broadband providers cannot charge companies for the privilege of allowing their websites to load faster — the essential components of what is known as net neutrality.

Though Mr. Wheeler’s proposal is not directly related to the Comcast-Time Warner Cable merger, those familiar with the F.C.C. say it signals a more vigorous approach to protecting customers, which could also affect thinking on the deal.

“What the F.C.C. is now apparently going to adopt signals a much harder line on their view of the state of competition in the broadband market,” Mr. Werbach said. “If their view is that the market is not working as it is right now, it’s less likely that they’re going to feel that a combination of two of the largest players is going to be in the public interest.”

If Comcast’s acquisition of Time Warner Cable were to be blocked by either agency, the industry could be thrown into tumult, with industry observers speculating that it would set off a flurry of new deal activity.

“I don’t think they would say so publicly, but I think everyone involved has to be at least considering a Plan B,” said Craig Moffett, an analyst with MoffettNathanson research. He recently lowered his odds that the deal goes through from 80-20 to 70-30, for factors including a changing regulatory environment and potential changes to how regulators define the broadband market.

Yet with all the uncertainty, it is impossible to know how the F.C.C. and Justice Department will rule. Regulators rarely telegraph their intentions, though in some past deals that have been blocked, regulators have sent subtle signals about how reviews were going; they have yet to do that during this review process.

Comcast is in touch with regulators almost every day, Mr. Cohen said, supplying them with information about business operations and plans for the integration of Time Warner Cable.

“There is nothing that we have heard as part of those contacts that suggest this transaction’s review is any different than our prior transaction reviews, when our deals have been approved,” he said.

Time Warner Cable executives also expressed confidence in the deal, saying on a recent conference call that they expect the closing of the merger with Comcast to occur “early this year.”

Comcast deploys a vigorous lobbying effort in Washington, spending $17 million on such efforts last year, which made the cable operator one of the biggest corporate campaigners in the country. In several instances, state and local officials, including Hawaii’s governor and Oregon’s secretary of state, have sent letters to the F.C.C. supporting the deal that were written almost entirely by Comcast employees. (Government officials first had asked Comcast for help in providing the information.)

As for the government’s own timetable, the Justice Department has no firm deadline. The F.C.C. has an informal agenda to rule by the end of March, but could extend that.

Consideration of the deal by the F.C.C.is expected to intensify next month, after the net neutrality vote. At the same time, both the F.C.C. and the Justice Department are weighing the antitrust ramifications of another proposed media merger, the acquisition of DirecTV by AT&T.

Even if regulators approved the Comcast-Time Warner Cable merger, they could demand onerous concessions from Comcast. If that were to occur, or if Comcast decided that new net neutrality rules would hurt the merged company, it could simply change its mind and refuse to complete the deal, which forgoes a breakup fee.

Mr. Cohen emphasized this point in an interview. “It is absolutely accurate that we have a very broad right to walk away from the transaction,” he said, without elaborating on what conditions could cause Comcast to walk away.

Yet he cautioned that there was no indication that regulators would ask for costly divestitures.

“There hasn’t been anything that we have heard at this point that has led us to believe that anybody is thinking about imposing overly burdensome conditions on this transaction,” Mr. Cohen said.

But if regulators did ask for some divestitures, Comcast is unlikely to simply abandon the deal. In Time Warner Cable, Comcast sees the opportunity to become a truly national provider of television, Internet and phone services, giving it unparalleled scale.

Amy Yong, an analyst with Macquarie research, said the importance of Time Warner Cable’s assets had only increased for Comcast in the last year, as competition from telecom providers like AT&T and new entrants like Google has grown fiercer.

Because there is no formal proposal, Mr. Cohen would not say whether the new net neutrality rules might make the deal less financially attractive to Comcast. Even if the proposal is approved, it will face months, and possibly years, of review and potential legal appeals. Before that process is complete, a new administration could change the rules yet again.

Comcast, meanwhile, is spending countless hours and millions of dollars planning to swallow Time Warner Cable, and brushing aside any suggestion that the regulatory winds are blowing the wrong way.

“There’s no demonstrable information out there that that is the case,” Mr. Cohen said. “The regulators haven’t talked, because they never talk.”

Michael J. de la Merced contributed reporting.