The head of the biggest union local in the nation's biggest post office says there could be a paralyzing mail strike if the U.S. Postal Service tries to eliminate Saturday delivery - and an estimated 30,000 workers in the process.

Moe Biller, president of the feisty (and big) New York local of the American Postal Workers Union, says the idea of 5-day mail service is a "strikeable issue." His big local dominates the N.Y. postal scene, and N.Y. is the pivot point for much of the nation's mail. It handles more letters and packages than Great Britain and processes a major chunk of the bills, pension payments and packages Americans get and send.

Although strikes by federal workers are illegal, a New York led postal wildcat in 1970 from mail throughout much of the country. It idled 220,000 of the 600,000 employees. It was ended only after President Nixon agreed to amnesty, a 14 per cent pay raise and sent in face-saving U.S. troops to get employees back to work.

Postmaster Benjamin Franklin Bailar told the Senate on Monday that the cost-price squeeze requires that the postal service drop Saturday service by 1978 and get a raise in the first-class stamp next year from its current 13 cents to 15 or 16 cents.

Eliminating Saturday service and a switchover to electronic transfer of funds was proposed recently by a blue-ribbon panel named by the President and Congress. Key House members, however, have warned the post office not to do anything until after Congress had studied the idea.

It is unlikely - mainly for political reasons - that Congress or the White House will allow Saturday delivery to die. More likely that a greater federal payment will be pumped into the Postal Service, the way it was before it tried to run itself as a half-business, half-public service.

It is also unlikely that the postal unions, with the best contract setup in government, will strike. They would rather have Bailar depart the scene, and see the public service aspect of the old Post Office Department return.

Threats of cutbacks and strikes have a way of getting locked into concrete. Bailer genuinely believes he must economize this way. And the unions are equally convinced he is carving up their services so it can be parceled out to private industry.

The situation today is somewhat similar to one in 1970. Both sides, the Postal Service and the unions, talked tough. But believed, at least at the Washington level, that a strike was the last thing that would happen. Those bargaining nuances, however, were lost beyond the Beltway.

Workers in New York, Chicago, Philadelphia and Houston hit the bricks. Congress, the Postal Service, unions and the public ought to keep an eye on this situation. If you think postal service is bad now, imagine a week or two with no service at all.