Capitol Hill's leading advocate of the right of Congress to veto agency rules and regulations said yesterday that President Carter exaggerated in claiming that an appeals court "ruled that the congressional veto over executive decisions was unconstitutional."

"That is not what they did," Rep. Elliott H. Levitas (D-Ga.) said of the Monday decision by the 9th U.S. Circuit Court of Appeals in San Francisco. The 3-to-0 ruling was the first to strike down any kind of legislative veto. Other decisions, including one by the U.S. Court of Claims in 1977, have upheld such vetoes. The Supreme Court has yet to rule.

Carter gave his disputed characterization of the 9th Circuit's decision in remarks to reporters in Plains, Ga. Neither a White House fact sheet, the Justice Department, nor the appeals court made any sweeping statements about the scope of the ruling, which involved a law giving the House of Representatives a one-house veto over certain administrative actions of the Immigration and Naturalization Service.

Many legislative vetoes are of the "1 1/2-house" type, meaning that a disapproval by the House or the Senate stands unless rejected by the other body. Others become effective only with a disapproval by both bodies.

The 9th Circuit did not specificallyaddress the validity of such vetoes. Instead, it struck down a particular one-house veto on this principal ground: it violates the separation of powers -- the fundamental doctrine devised by the framers of the Constitution to prevent any of the three branches from systematic poaching on the governmental preserves of another.

The Justice Department will open staff discussions Monday on whether to ask the Supreme Court to rule in the half-century-old constitutional power struggle between the legislative and executive branches of government over congressional veto power over administrative actions.

But, Levitas, in a telephone interview, raised the possibility that bipartisan congressional leaders will ask the department to let the incoming Reagain administration decide whether to appeal.

The Republican Party platform and the president-elect have endorsed the legislative veto although it has been opposed by all eight presidents since Herbert Hoover.

"The unremitting delegation of authority to the rule-makers by successive Democratic congresses and the abuse of that authority has led to our current crisis of overregulation," the platform said. "For that reason, we support use of the congressional veto. . . ." Ronald Reagan, in a campaign speech in Youngstown, Ohio, in October, took a similar position.

By contrast, Carter, expressing views in line with those of the seven chief executives who immediately preceded him, hailed the 9th Circuit. Its holding that the House veto in the immigration law is unconstitutional has "perhaps the most profound significance constitutionally of anything that's happened in my four years" in the White House, he said in his remarks to reporters.

The White House face sheet said that the 9th Circuit's reasoning "confirms the view . . . that the legislative veto provisions of many regulatory statutes, especially those authorizing a veto by resolution by only one house, are unconstitutional." But the fact sheet also was careful to say that the appeals court "doesn't expressly decide the question of whether legislative vetoes issued under the statutory authority granted by Congress -- as distinguished from vetoes of quasi-judicial executive branch actions such as a deportation order -- would be permissible under the Constitution."

The decision dealt a blow to the House and Senate, each of which had been requested by the 9th Circuit to file a friend-of-the-court brief.

Both bodies argued that one-house vetoes -- which proliferated in the Carter administration -- are shielded by a provision of the Constitution empowering Congress "to make all laws which shall be necessary and proper" for the execution of its legislative mission.

But Judge Anthony Kennedy dismissed the argument in a brief section of his 45-page opinion for the panel. "We note that [the provision] authorizes Congress to 'make all laws,' not to exercise power in any way it seems convenient," he wrote. "That a power is clearly committed to Congress does not sustain an unconstitutional form in the exercise of the power."

At Carter's request, the Justice Department, even though it won in San Francisco, will consider whether to appeal to the Supreme Court in order to obtain a resolution of the issue.

Precedent exists not only for appealing in such circumstances, but also for appealing in order to enable the House and Senate to argue their position. They cannot appeal because they are not parties to the case. An appeal in a case of this kind must to taken by the high court -- although review by the justices in no way forecasts how the case ultimately would be disposed of.

(The parties are the Immigration and Naturalization Service and Jagoish Rai Chadha, whom the INS tried to deport. Although the Justice Department agreed with Chadha's counsel, Public Citizen's Public Litigation Group, that the House resolution vetoing an INS decision to let remain in this country was unconstitutional, the 9th Circuit held that there was a genuine controversy to be litigated, as required by the Constitution.)

Levitas said that Congress as well as the administration want the legislative issue resolved, but that the key question is whether the case "frames the issue as precisely as we want it framed in order to get a ruling on the constitutionality of the congressional veto." The issue is framed much better by a natural gas regulation case pending in the U.S. Court of Appeal for the District of Columbia, he said.