Rival postal unions have brought several thousand legislative shock troops to town - and coincidentally, under the same hotel roof - for a faceoff that spells trouble for many liberal members of Congress.

The issue is what, if anything, Congress wants to do about the predominantly black National Alliance of Postal and Federal Employees. It has been trying to get the courts, federal agencies and now Congress to increase its clout by giving it the right to represent members and postal employees in grievances with the U.S. Postal Service.

Under present rules, the independent union can represent members or employees only in discrimination cases because it lacks the national recognition won and enjoyed by larger AFL-CIO rivals who represent the majority of the nation's 550,000 rank-and-file postal workers on craft lines.

Yesterday, the House Post Office Civil Service Committee failed to vote on the so-called Alliance Bill. The non-action saved, for the time being, liberal members who don't like being caught between the AFL-CIO, which backs its unions, and key civil rights groups, who favor the alliance.

After taking action on three other bills, the committee, headed by Rep. Robert N. C. Nix (D-Pa.), adjourned so that members could attend a scheduled meeting of the Democratic Caucus. Alliance leaders believe their bill was kept until the last minute so the adjournment could be ordered.

Alliance president Robert White called the action "a breakdown in the legislative process." Nix, who hasn't taken a public stand between the alliance and the AFL-CIO group, is black and is running for reelection this year.

Alliance leaders contend that Congress and the USPS have sold them out to more powerful AFL-CIO groups. They have attempted to make the issue one of civil rights as much as union representation. Most of the union's 22,000 members are black. And most are in the postal service.

AFL-CIO groups assert that, in both numbers and service, they do a better job of representing black postal workers than does the alliance.

Their leaders say the indendent union has failed in the courts, and now is trying to get Congress to carve out status for it that it has not earned. One of the unions, the American Postal Workers Union (APWU), estimates that 75,000 of its 250,000 members are black, and says that six of 22 national elected officers, including its vice president, are black.

The key sales pitch that federal and postal unions can make in recruiting and retaining members is the fact that they have the talent, money and authority to represent workers in grievances. The alliance, because it lacks national exclusive bargaining rights cannot do this and therefore tackles every grievance case as a discrimination issue.

By coincidence, both the alliance and two of its rival AFL-CIO unions had rallys yesterday in the same place, the Sheraton-Park Hotel. The APWU and the National Association of Letter Carriers sent many members to Capitol Hill to urge the committee to kill, or delay, the alliance bill vote. The alliance also had lobbyists working the halls. After the adjournment, the committee agreed tentatively to take up the legislation on Feb. 4.

Meantime, many liberal members of the committee and liberal members of Congress are wishing the issue would go away. Alliance leaders believe they can win their bill, if Congress votes on it.

"This thing presents the classic dilemma for liberal members," a House staff aide noted. "The alliance is trying to make this civil rights, race issue. The AFL-CIO is saying 'look, we represent labor and we are your friends, vote with us.' Both sides represent constituencies that nobody up here wants to offend."