But the Democratic nominee for the White House may not be the only one adversely affected.

Sinclair - the nation's largest owner of television stations, many of them in electoral swing states - is itself running a significant financial and political risk by telling its stations to pre-empt regular programming and carry the film. Already, Sinclair's decision has alienated some advertisers; enraged consumer and media watchdog groups, who are vowing to challenge its station licenses when they come up for renewal; and given pause to some analysts and investors considering the company's financial outlook.

Representatives of Sinclair did not return repeated phone calls seeking comment. But the company, whose executives have been among the largest media contributors to President Bush, has said the documentary deserves to be seen because it is news, and as such does not fall under federally mandated equal-time provisions for political candidates.

In the film, "Stolen Honor: Wounds That Never Heal," former prisoners of war in Vietnam call Mr. Kerry's 1971 Senate testimony a betrayal that prolonged their captivity. In that testimony, Mr. Kerry quoted other veterans talking about American atrocities.

The Kerry campaign has called the film a politically motivated attack that is unfair and inaccurate.

Last week, the Kerry campaign formally asked Sinclair for equal time to respond to the film, a move that could lead the Federal Communications Commission to consider whether to order the stations to allow a response.

Sinclair is no stranger to political controversy. In April, its eight ABC affiliates pre-empted the "Nightline" program in which Ted Koppel read aloud the names of American soldiers killed in Iraq. Sinclair declared the show "unpatriotic" and harmful to the war effort, adding that "Nightline'' was motivated by an antiwar agenda; it instead offered its ABC affiliates a special that debated the issue.

Earlier this year, Mark Hyman, the vice president for corporate relations at Sinclair who doubles as a conservative commentator on the company's newscasts, took a crew to Iraq to find what he called the untold positive news of the war. And after the attacks of Sept. 11, 2001, Sinclair ordered newscasters at its Fox affiliate in Baltimore to read patriotic statements praising President Bush.

But the furor over "Stolen Honor'' appears to be affecting Sinclair in ways these previous actions did not. In some cities - among them, Portland, Me.; Madison, Wis.; Springfield, Ill.; and Minneapolis - local advertisers, including car dealers, furniture makers, supermarkets and restaurants, have taken their commercials off the company's stations.

"I've decided I don't want to advertise on them," said Adam Lee, the president of Lee Auto Malls, which owns 10 auto dealerships in Portland Me., and has ordered its advertising off the CBS affiliate, WGME. "It's a public trust. It seems they're abusing it. If it were a news show and they were really trying to do a fair and balanced story on both sides, that would be a different matter. I don't think they are. That's not their intention.''

Groups, including Common Cause, the Alliance for Better Campaigns, Media Access Project, Media for Democracy and the Office of Communication of the United Church of Christ, are putting together a database listing all Sinclair advertisers and will try to persuade others to withdraw their commercials. Among those on the list are chains like Applebee's International, Best Buy, Chili's, Circuit City, Domino's Pizza, Lowe's, Papa John's, Subway, Taco Bell and Wal-Mart Stores.

The groups are also vowing to find groups in cities with Sinclair stations who will challenge the broadcast licenses of every Sinclair-owned station over the next several years. Such challenges almost never result in lost licenses, but they often result in heavy legal costs for the station having to defend them.

In addition, some analysts said Sinclair might have hurt itself in the continuing battle over loosening media ownership rules, a fight in which Sinclair has been a leader. Efforts at further deregulation were stalled this year when the United States Court of Appeals for the Third Circuit, in Philadelphia, ordered the F.C.C. to reconsider its relaxation of such rules.

A report issued by the firm Legg Mason last week cited the controversy over the film and asked the question, "Is this good for investors in terms of increasing the odds for favorable deregulation?" The conclusion: "We think not."

Blair Levin, the managing director of Legg Mason and a former F.C.C. official, added in a telephone interview, "Deregulation usually happens when you do it quietly."