The nation's half million postal workers hava a big problem. In 10 days it could be everybody's problem.

The contract between the U.S. Postal Service and its AFL-CIO unions expires at midnight, July 20. At the moment it appears the unions may not get what they believe they must have.

If union's don't get what they want - a 14 percent first-year raise, faster cost-of-living payouts and lifetime job guarantees - there could be a strike.

Isolated walkouts a la the 1971 surprise wildcat, could cause moderate to serious delays in delivery of everything from back-to-school catalogs to Social Security, welfare and pension checks. A widespread strike, hitting cities like New York and Chicago, could cripple mail delivery everywhere.

No-contract - no work talk from New York area members is serious. New York is the home of the three "Ms" militancy, mouth and muscle. It also is the biggest mail clearing house in the world, and strike talk also is coming from Philadelphia, San Francisc, Houston and St. Louis union leaders.

Carter aides, without success, have asked union's to hold wage demands within the magic 5.5 percent area. Even that "low" figure would cost the USPS nearly $1 billion a year. So far, White House jawboning has done nothing but infuriate unions and anger postal officials. They are particularly touchy about their corporate-status independence now that portions of Congress and the public are demanding a return to the "good old days" of a politically-sentitive, heavily subsidized postal service.

Postal workers, who enjoyed considerable public sentiment several years ago, have lost a lot of it. The White House lid on other federal salaries, the recent stamp increase and taxpayer revolts have eroded much of the good will the mailman traditionally has received.

The nation's media generally has been unfriendly to postal worker demands. Often times it has confused botched delivery programs, price increases and service cutbacks with rank-and-file people who sort, move and deliver the mail according to master blueprints drawn up in Washington.

Labor costs in the postal service consume 85 cents of every dollar the USPS makes, but many of the bills coming due now are the result of pie-in-the-sky programs dreamed up by USPS executives now in retirement.

Recently the General Accounting Office blasted the billion-dollar bulk mail system, GAO said, in effect, it doesn't work as well as old methods of routing and sorting. Ironically, a Washington-based postal official recently sweated for 10 days awaiting a letter mailed 10 miles from his home. Somehow, moving from one Washington suburb to another, it first passed through a New Hampshire post office. He's still a believer, but a shaken believer.

Unions say that White House "whiz kids" running the nation's economic show fail to realize that the postal service is a dead-end employer, offering little chance for advancement. Seniority pay, and job tenure, the unions believe are vital.

White House aides say somebody has to take the first step in accepting voluntary wage-price restraints. The fact that coal miners and railroad workers have not is, they believe, all the more reason the postal contract must be kept "reasonable." Postal union leaders say that is all the more reason to get as much as they can.

Postal union leaders are in a bigger bind than they care to admit. This has a major bearing on the contract talks. The two biggest unions hold their conventions only weeks from today - after the contract expires.

J. Joseph Vacca, head of the powerful Letter Carriers union, faces his Chicago convention delegates for reelection only nine days after the contract is up. A "good" contract should sweep him to victory. A "bad" one could put him back in Cleveland carrying the mail.

Emmet Andrews of the American Postal Workers Union has more breathing space and a different set of problems. His opposition, so far, is from Harold Stassen-like candidates who say they run mainly to give union voters a choice.

Andrews and other APWU officers are up for reelection via a mail ballot. Ironically, for them, a postal strike could mean that the 230,000 ballots might handled by supervisory personnel and Army troops who would be called on in event of a serious walkout.

There is still time for a face-saving, strike-avoiding contract. That time, obviously, is running short. Big mailers are worried about the prospect of a strike. The Social Security Administration is worried. USPS officials, union leaders and postal workers and patrons are worried.

It is easy - but dumb - to dismiss strike talk as bargaining table bravado. It may be that, but a lot of in-the-know people realize that talk can be translated into action. Nobody in authority is urging people to mail early for Christmas, but a Christmas-type rush in July is something to think about.