Former Washington Star owner Joe L. Allbritton has made an offer "in excess of $100 million" to buy the New York Daily News, sources said today.

Sources said the offer to the Tribune Co. of Chicago, which owns the Daily News, was the second made by Allbritton. His initial offer was rejected.

A spokesman for Allbritton Communications Co. in Washington declined comment. Ira Harris, managing director of Salomon Bros. Inc., who is handling negotiations for the Tribune Co., also would not comment.

It is understood that Allbritton's offer to the Tribune Co. is contingent on his getting generous concessions from the News' 11 powerful trade unions. Sources familiar with negotiations question whether the unions would agree to the concessions necessary to keep the News in business.

One estimate is that to be economical the newspaper's 4,500-member work force must be reduced by 1,300 to 1,500 employes.

Sources warn that it is difficult to put a precise figure on the Allbritton offer because the deal could be structured in a variety of complex ways. These sources add, moreover, that any offer would include not only the newspaper's assets, but also its liabilities, which reportedly are substantial.

Sources said today that central to any deal is the News' most valuable asset, the 37-story art-deco News building on East 42nd Street and its 11-story wing. Experts familiar with Manhattan's real estate market say the building, with 1.1 million square feet of space, is worth between $100 million and $135 million.

Other elements of the News package, most of which are described as "marginally valuable at best to the News' survival," include Newspoint, a plant on about 35 acres on the East River in Queens where the newspaper's comics and Sunday magazine are printed; a plant located on three acres in Brooklyn; and Gaynor News Co., which distributes papers in the suburbs.

The Tribune Co., a communications and entertainment conglomerate, announced Dec. 18 that the New York tabloid--which has the nation's largest general news circulation--was for sale. At the time, the Tribune Co. predicted that the News would lose about $11 million in 1981, its first losing year.

Earlier this week, an executive at Warner Communications Inc. disclosed that his company had abandoned interest in the News. He said that Warner could not approach the unions from strength if it was not "the buyer of last resort."

A source today described Allbritton as "the buyer of last resort," and said he seemed determined to acquire the News even in the face of its financial and union problems. Besides, said this source, Allbritton does not have to account to stockholders as must executives at a publicly owned company like Warner.

"Ego has to play a role in his decision to take this one on," said one knowledgeable source.

In recent years, the News' circulation had dropped, a trend the paper has halted largely by daily promotional games with cash prizes. The paper's income is also threatened by the rocky financial condition of some of its former major retail advertisers, according to sources. But Allbritton's biggest concern by far has to be the unions.

For example, the News' contract with the unions includes a "me-too" clause. According to most interpretations of that clause, if the unions make concessions to the News, the same concessions are automatically extended to the rival New York Times and Post.

The News also has life-long contracts with some 650 individual employes who lost their jobs to automation.

The ages of those employes range up to 83, according to sources familiar with the News. Some 60 stereotypers have no jobs at all at the paper. Many of these employes with lifetime contracts spend their required 4-hour, 40-minute workday in what is called the "rubber room" at the News, playing rubbers of bridge