When the Tribune Co. of Chicago said on Dec. 18 that its New York Daily News was up for sale, it sounded like the death knell for yet another newspaper.

But more than three months have passed and the Daily News is far from dead. In recent days, Donald Trump, a 35-year-old Manhattan real estate mogul, has emerged as the new front-runner to acquire the newspaper, bypassing former Washington Star owner Joe L. Allbritton, who remains in the competition.

And with each passing day it becomes more and more difficult for the Tribune Co. simply to shut down its money-losing offspring, as the owners of other failing newspapers have done, without suffering a public relations disaster.

White knights spring up almost daily with the professed goal of saving the ailing tabloid. Also aligned against the Tribune Co.'s implied threat to close the paper are the leadership of the paper's craft unions and a variety of state and city officials. There is even a fifth column movement growing within ranks of Tribune Co. shareholders.

According to management sources at the Daily News at least three would-be buyers have presented the Tribune Co. with proposals to take over and operate the Daily News--Trump, Allbritton and John S. Dyson, 38, chairman of the Power Authority of New York State, the largest nonfederal public power agency in the country. None of the principals would comment on the negotiations.

When Trump's name first surfaced, it was assumed that he simply wanted the Daily News building on East 42nd Street, a property valued between $100 million and $135 million, according to real estate experts.

But two separate sources say that Trump told the Tribune Co. that he would not seek to buy the building. Instead, he reportedly wants a long-term lease on the part of the building used by the newspaper, cancelable if the paper fails or moves to another location.

Dyson, a Democrat who has a great deal of family wealth behind him, has run unsuccessfully for Congress and was considering running for governor before Mayor Edward Koch announced. Before he was power commissioner, he was commerce commissioner and is credited for being the architect of the famous "I Love New York" advertising campaign.

But what would a buyer get for his money? Except for the 42nd Street building, the answer has to be a lot of headaches.

For one thing, if the newspaper folds, estimates of the severance pay that will be owed employes range from $40 million to $60 million. Another elusive figure, which is called frightening in its potential magnitude by one source who is familiar with the Daily News' books, is the shortage in the pension fund assets. Under federal law, if a company goes out of business, pensions must be funded to a certain level.

The complex questions about how these obligations will be met, along with the News' aged plant in Brooklyn and its modern, but unnecessary Sunday magazine printing operation in Queens, are part of the discussions, according to sources.

Another issue is whether the new management would continue to buy newsprint from the Tribune Co.'s mills. This is an important concern for the Chicago company since the Daily News, with 1.5 million daily circulation, 2 million Sunday, is one of the largest consumers of newsprint in the world.

But the unions remain the biggest unknown to any would-be buyer of the Daily News. The coalition of unions representing 3,800 full-time employes are also playing the white knight role.

On Thursday, Tribune Co. management met with union officials for the first time since the sale was announced. After the 3-hour session, a union official claimed that unidentified executives "made it clear that the unions would have a full opportunity to explore all alternatives for the survival of the Daily News."