Rep. Mike Synar (D-Okla.) filed suit yesterday challenging the new balanced-budget law, just hours after President Reagan signed it, on grounds that it violates "the lawmaking procedures of the Constitution."

Synar filed the suit in U.S. District Court here under special provisions of the act allowing for an expedited court test. He aimed his complaint at the "trigger mechanism" in the measure giving three federal agencies authority to determine whether government spending must be reduced and what cuts must be made.

"I support a balanced budget," Synar said at a news conference. "And I believe that dramatic action is needed to achieve that goal. But I also deeply believe that there are basic principles underpinning our system of government which cannot be violated."

President Reagan noted the issue yesterday morning as he signed the legislation. He said he was mindful "of the serious constitutional questions" raised by the triggering provisions and hoped they would be resolved promptly.

"What we did was turn over the whole budget process to a bunch of unelected bureaucrats," Synar said. "We cannot legislate leadership. The drafters of our Constitution believed Congress should make these hard choices because Congress is the branch of government closest to the people."

Synar and his chief lawyer, Alan B. Morrison, head of the Public Citizen Litigation Group, pointed out that the law provides for a fallback procedure in case any part of the triggering mechanism -- which relies on the White House Office of Management and Budget, the Congressional Budget Office and the General Accounting Office -- is held invalid.

The fallback method would still allow OMB, an arm of the executive branch, and CBO, an arm of Congress, to determine each year whether specified reductions in the federal deficit are being met and, if they are not, to formulate a list of further cuts in government spending to meet the goals. But the GAO, which as the law now stands has final authority to tell the president what cuts to make, would be out of the process.

Instead, the OMB-CBO's report would be submitted to a temporary joint committee of Congress. The committee would introduce the report as a joint resolution that would have to be approved by both houses of Congress and be signed by the president. The procedure therefore would be similar to the current budget process except that amendments might be more difficult.

Synar, a House conferee on the Gramm-Rudman-Hollings balanced-budget measure who refused to sign the conference report, said he was satisfied that the fallback system would be constitutional.

"In other words, each fiscal year, if Congress has failed to meet the deficit targets, the CBO and OMB would report that fact to Congress," Synar said. "Each house then would have to vote up or down whether to allow the sequestration budget-cutting process to go forward."

Morrison, who has assailed the Gramm-Rudman-Hollings measure as "government by gimmickry," said the suit was filed now in hopes of winning a quick decision by a special three-judge panel in U.S. District Court and then a final ruling by the Supreme Court before next March 1. That is when the first "sequestration order" by President Reagan is supposed to take effect.

"It does no good to wait until March 1 and have all the confusion of a court case when we're trying to establish a sequestration order," Synar said. He said he expects lawmakers of both parties to join him in the suit next week.

The case was assigned to U.S. District Court Judge Oliver Gasch, who will be joined by two other judges yet to be named. Under the act, their decision can be appealed directly to the Supreme Court.