Consumer advocate Ralph Nader, expressing strong disillusionment with President Carter and reservations about Carter's possible Democratic challengers, said yesterday that the time is coming for a new political party in the United States.

Nader said it was not likely the movement would "get very far" in the 1980 presidential campaign, but said it could be a major force in local and national politics by the 1984 presidential campaigns.

Declaring the "two-party system is bankrupt," Nader said he thought the new party could combine the resources of consumer groups, antinuclear organizations, community-based citizens organizations and some of the major trade unions.

He said the new movement would focus on "the overriding issue of our times - corporate power - " and what he called "the expansion of citizen access to all branches of government, the mass media and corporate decision-making."

Nader, who has declined past suggestions that he run for political office himself, did not specify what role he would play in creating a thrid party. But he said at a luncheon with a group of reporters that "most of our work" in the coming year would concern the "citizen politics which must be done before a new politics can arise."

Public Citizen, which is Nader's blanket organization in Washington, already has begun organizing activist groups in target congressional districts, and on Sunday. Nader was one of the major speakers at an antinuclear rally that drew at least 65,000 people to Capitol Hill.

In 1976, the well-known consumer advocate lent his presence and implicit support to the Carter campaign. While stopping short of a formal endorsement, he visited Carter in Plains, Ga., and pronounced the Democratic nominee's positions "more admirable" than those of any other candidate in decades.

But yesterday he said that the last eight months have seen a "complete corporatization of Jimmy Carter," exemplified by decisions on nuclear policy, decontrol of oil prices, and opening of national forests to commercial use. These decisions more than offset the gains resulting from Carter's appointment of many public advocacy activists to administration posts, Nader said.

"He has made no effort at national leadership in the FDR style," Nader said of his onetime favorite. "He has not spoken out. He does not appreciate the dimensions of national leadership. Simply to mumble your way through two years is to suggest you're not comfortable with the role of presidential leadership."

As for California Gov. Edmund G. (Jerry) Brown Jr., Carter's most likely Democratic challenger in the 1980 primaries, Nader said, "I don't think he'd be that much different from Carter." Noting that Brown spoke at Sunday's antinuclear rally, Nader said "he has been very good" on that issue. But he said that Brown, like Carter, is a "very cautious" politician, with a "strong feeling of what the corporations can do to a politician" who defies their wishes.

Nader said that Sen. Edward M. Kennedy (D-Mass.) "has a little more courage" than the other two, and could "call the bluff better" when corporations challenged his policies.

But, overall, he said, the "choices among the Democrats are very narrow," necessitating the formation of a third party that could elect a president who was a product of "a real mass movement."

Nader also said that he favored the idea of holding a constitutional convention "before the end of the century," to rewrite the Constitution and deal directly with the question of corporate power which was unknown two centuries ago. CAPTION: Picture, Nader: Carter "does not appreciate the dimensions of national leadership." By James K. W. Atherton-The Washington Post