Montgomery County's cable television company, in another major blow to cable-hungry subscribers, has proposed slashing up to half of the 120 channels of programming it promised to deliver when it agreed to build the county system two years ago.

The change, which would constitute another major retrenchment of the system, was contained in a formal, 16-page request for contract modifications that was released yesterday by Tribune-United Cable Co.

The company now offers about 70 channels of programming to 18,000 subscribers. It plans to broadcast eight hours daily of programming on the county's public access channels and to begin regular programming on the county government channel Nov. 20.

A Tribune-United spokesman said yesterday that some channels currently offered to subscribers may be eliminated if the county grants the company's request to slash the number of channels.

Invoking a year-old federal law designed to bail out financially troubled cable systems, the company said that many of the proposals contained in its contract had become "commercially impracticable."

"This unfortunate development is precisely the sort of problem the federal law was designed to address," the request said. "Congress was clearly responding to a growing recognition that inflexible franchise agreements were threating the viability of cable systems . . . . "

The request, the latest development in Montgomery County's growing cable woes, could be a significant factor in the company's effort to modify substantially the 100-page contract it negotiated with the county in 1983.

The contract calls for Tribune-United to deliver more than 100 channels of programming, plus a range of services such as public access channels, government and educational channels, public studio use, an institutional network and millions of dollars in grants for local programming.

The company, as part of a nationwide trend, revealed last month that it was attempting to get out of its commitments, citing potential losses in excess of $100 million. The county has steadfastly refused to void the contract, although it has agreed to discuss specific issues and has renegotiated some contract items.

Tribune halted all construction on the system last month, delaying indefinitely the delivery of cable services to thousands of county residents. The county notified the company that it had 10 days to resume work or face stiff fines, which prompted the filing of the request, said Tribune-United spokesman Michael Pohl.

Jay E. Ricks, an attorney with the law firm of Hogan and Hartson, which is representing Tribune-United, said yesterday, "This may be one of the first cases brought under this federal law. We're not aware of any other case that has been decided."

Under the law, Ricks said, Tribune-United needs only to demonstrate that the contract provisions are economically unfeasible. The county would have 120 days after that to negotiate and agree to modifications.

If the changes are unacceptable to the company, Ricks said, Tribune-United would have the right to sue in federal court and ask a judge to modify the franchise agreement.

"This goes considerably beyond contract law," he said. "It recognizes that in the whole process of cable franchising there was traditionally encouragement from local governments to bid the outer limits of what might or might not be available."

County lawyers were scheduled to brief the County Council on the latest developments this morning in a closed session. Most county officials contacted were reluctant to comment on the situation.

"My feeling, generally speaking, is when a contract is made in good faith it should be lived up to," said council President Michael Gudis. "But I really can't discuss any of the details."

In the request, the company asks for a "comprehensive review" of the contract and specifically asks for relief from promises to provide public access programming grants and to build an institutional network.

Saying that the financial difficulties were largely beyond its control, the company is asking for a modification of its construction schedule and the channel reduction.