An attempt to save The Washington Star by offering extraordinary federal tax incentives to any purchaser of the city's oldest newspaper appeared to collapse almost as quickly as it arose yesterday after Sens. Charles McC. Mathias (R-Md.) and John Warner (R-Va.) abandoned their attempt to attach a rider to the nationwide tax-cut bill.

"It's not going to work," said Mathias, appearing with Warner at a press conference in the Capitol. Warner said they made "a good faith effort, but time ran out" as the tax bill went to a House-Senate conference without the rider.

That leaves the 128-year-old Star with no source of rescue in sight and six days remaining before its scheduled to close because of continual operating losses and declining circulation.

The tax meauser, drafted to apply effectively to only one newspaper in the country, would have permitted any new purchaser of The Star to recover up to 96 percent of its business losses over the next three years -- thereby removing the biggest single obstacle cited by potential purchasers. The Star was reportedly losing $20 million a year under its present ownership, and under current tax laws, only 46 percent of those losses could be recovered.

Chris-Craft Industries of New York, which might have purchased The Star if the tax break had been enacted, certainly will not do so without it, Herbert J. Siegel, chairman of the conglomerate, said yesterday. w

"You'd have to be out of your mind," he said.

The only person known to be still contemplating a bid is Boston real estate developer Mortimer B. Zuckerman, owner of The Atlantic Monthly. Zuckerman said yesterday he was "still trying to bring myself up to speed on various things, including the value of The Star's assets," before deciding what to do.

R. Robert Linowes, former president of the Greater Washington Board of Trade, who has been trying to save the paper through a partnership between employes and investors, said "Things are going on, but nothing we can talk about now."

Linowes said it was an "amazing coincidence" that a possible bid for the paper should have come from Chris-Craft, where his brother David is a memeber of the board of directors. Neither he nor his brother, he said, knew anyhting about Siegel's attmept to gain tax incentives that would have made the ailing paper a feasible investment.

A spokesman for Time Inc. said there was nothing new to report about the search for a buyer, and nothing is likely to happen over the weekend. That would take The Star into what would probably be the last week of its life.

The Mathias-Warner press conference put a quick end to an effort that was apparently doomed from the start.

Mathias said that various proposals to help save The Star were considered.

The proposal that collapsed yesterday might have been attached to the tax bill before the Senate sent it to the conference yesterday to resolve differnces between the House and Senate versions of the $750 billion omnibus tax cut.

But, Said Warner, whose role in this rescue effort had not been disclosed earlier. "That would have required unanimous consent [of the Senate.] I leave it to your collective judgement whether that could have been obtained."

Warner and Mathias said they tried to draft a measure that would have benefited The Star without appearing to aid a specific corporation. It would have had to "accomodate any number of interested parties," Warner said, without offering tax sweetners to every newspaper in the country. The were aided by Ronald Lewis, a tax lawyer in the law firm headed by a former Internal Revenue Service commissioner Mortimer B. Caplin, who was brought into the discussions by Chris-Craft's lawyer J. Stanley Pottinger, a former assistant U.S. attorney general.

In addition to the technical and parliamentary problems, Warner said, there was a political problem. Federal tax breaks for a newspaper "would have required a great deal of debated," he said, "and would even have riased constitutional issues." After dicussions with the Senate leadership, he said, he and Mathias concluded that their effort was futile.

It is theoretically possible for the rider to be attached to the tax-cut bill during the House-Senate conference, which began yesterday and was expected to last at least one more day. However, such a move would require extraordinary and elaborate pariliamentary maneuvering that no one is attempting. Asked if congressional action on The Star was out of the question, Warner responded, "Yes, out."