The American Civil Liberties Union threw itself a party in a Dirken Senate Office Building hearing room yesterday afternoon, but the atmosphere seemed something less than triumphant.

Although he wasn't there, Sen. Jesse Helms (R-N.C.) made his presence felt.

"There's a climate of siege on the Bill of Rights, particularly in the Senate right now," said John Shattuck, the ACLU's national legislative director."We're going through a period of virtual constitutional revolution where a minority of senators, and a minority of the people, really, are dictating the agenda. The people voted to change the president, not the Bill of Rights."

Helms and Sen. J. Bennett Johnston (D-La.) are leading the current fight in the Senate to limit the powers of the federal courts to rule on social issues such as school desegregation, and they appear to have the votes needed to overcome opposition from Sen. Lowell Weicker (R-Conn.) and others.

The reception capped off several days of lobbying efforts by members of the National Board of the ACLU in support of legal services, the extension of the Voting Rights Act and reproductive freedom.

The ACLU, said Shattuck, agrees with Weicker's contention that the question is not of desegration but of constitutional law. The debate centers on a series of amendments and counteramendments to a Justice Department authorization bill now before the Senate.

Weicker, appearing at the reception shortly after the day's floor fight, looked weary even while he tried to sound optimistic. "There's not a person out there -- conservative or liberal -- who, if they saw that it is the Constitution that's under attack, wouldn't say, 'Whoa, wait a minute.' People will come to understand the issue is not busing or abortion but the Constitution," said Weicker, one of 10 congressmen co-sponsoring the ACLU's reception. "Obviously we don't have the votes, but I don't know what we can do but take it one day at a time."

Sens. Spark Matsunaga (D-Hawaii), Carl Levin (D-Mich.) and Charles Mathias (R-Md.) all expressed agreement with Weicker's position, but none felt that he could defeat Helms' efforts to limit the powers of the federal courts. "It doesn't look very good. I would think the Helms amendment will pass," said Levin.

Although the conservative voice of Helms has many ACLU supporters on the defensive and in a pensive mood, another southern accent provided at least one moment of levity at the reception. Said one of Sen. Howell Heflin's (D-Ala.) summer interns as she perched over long trays of Brie, cheddar, apples and pears: "I don't know what this party's for, but I'm just starved!"