Perhaps no constitutional matter is more vigorously policed in turf-conscious Washington than the separation of powers, by which the founding fathers sought to divvy up authority among equal branches of government.

So it is that a provision in the 1984 Deficit Reduction Act has become the latest centerpiece in a battle between Capitol Hill and the White House, with each side accusing the other of usurping its rightful authority.

The provision gives the comptroller general of the United States, who is head of the General Accounting Office, authority to hold up a federal contract if a competitor has filed a legitimate protest over the bidding process. Previously, the GAO had authority to look into bid protests but no power to keep a contract from being awarded during an investigation.

In effect, the 1984 law gave the GAO that power, which Congress figured was a good way to keep agencies from improperly awarding contracts.

Last July, President Reagan vigorously objected to the provision, arguing that the comptroller general is an "officer of Congress" who could not interfere with executive branch procurement.

Reagan signed the bill into law anyway. But in December, armed with a Justice Department opinion that the provision was unconstitutional, the Office of Management and Budget instructed departments and agencies to ignore it.

That has rankled key members of the House and the Senate. They contend that Reagan either should have vetoed the bill or honored the law until the courts had ruled on its constitutionality.

At a congressional hearing last week, Rep. Jack Brooks (D-Tex.), chairman of the Goverment Operations Committee, lashed out at OMB Director David A. Stockman for going "so far as to order federal officials to willfully violate the law."

"Absent a judicial determination on the provisions in question, it is the responsibility of government officials to comply with the statute in its entirety," Brooks said. "I believe the president has received incredibly bad advice from the attorney general [then William French Smith], the OMB director and the major procuring agencies on this matter."

In a statement at Brooks' hearing, Sens. William S. Cohen (R-Maine) and Carl Levin (D-Mich.) said that Reagan's "unilateral decision . . . to refuse to enforce a statute constitutes a usurpation of the proper role of the judiciary and a failure by the president to meet his constitutional responsibility to 'take care that the laws be faithfully executed.' "

Legal opinions abound on the controversy. Former attorney general Smith said in a recent letter to House Judiciary Committee Chairman Peter J. Rodino (D-N.J.) that the administration's position is based on a 1983 Supreme Court decision that threw out the legislative veto (Immigration and Naturalization Service v. Chada).

The legislative veto had become an increasingly popular device. Lawmakers saw it as a means to second-guess executive branch decisions. According to Smith, giving authority over contracts to the head of the GAO, "an arm of Congress," amounted to the same usurpation of veto power.

But the American Law Division of the Library of Congress, in an opinion requested by Brooks, essentially sided with the lawmakers.

The law division noted that the Constitution does not give the president the power to suspend a law.

"The legislative nature of the action taken by the administration . . . seems clear," the opinion said. "In doing so, the president has effectively suspended its operation across the board, an action which entails lawmaking."

By basing his order on the alleged unconstitutionality of the provision, the law division said, Reagan "arguably" intruded on the power of the judiciary as well.

The murky state of the legal opinions is due in part to the murkiness involving the status of the GAO: Is the comptroller general an officer of the legislative branch, the executive branch, or both?

The administration argues that the GAO is beholden to Congress, not to the White House, and that the comptroller general does not serve "at the pleasure of the president." But Comptroller General Charles A. Bowsher, who holds a 15-year fixed term, reminded Brooks' committee last week that he was appointed by the president and confirmed by the Senate.

Brooks' committee is looking for a parallel between Reagan's action and that of President Andrew Johnson, the only president to be impeached. Johnson, who was narrowly acquitted in a Senate trial in 1868, was accused of violating the federal Tenure of Office Act by attempting to fire his secretary of war, who had sided with Johnson's congressional adversaries.

The American Law Division did not find a clear precedent in the Johnson impeachment. It noted, however, that Johnson's supposed offense "occurred in justifying his exercise of a veto, not in defiance of a validly enacted and effective law."

Brooks said he intends to question Stockman and Justice Department officials at a second hearing today.