The Rev. Jesse L. Jackson, flanked by black elected officials, peace activists, environmentalists, farmers and progressive labor leaders, opened the first convention of his national Rainbow Coalition yesterday by declaring, "We are the new majority in this nation."

Jackson, who coined the term Rainbow Coalition when he placed third in the race for the 1984 Democratic presidential nomination, said he wants to fashion a "permanent progressive political organization" of groups whose interests have been ill-served by the two major political parties.

But he made clear that he considers his group "enlightened Democrats," determined to make their party honor its commitment to "humane policies at home and human rights abroad."

"We will not sit idly by and watch the party shift to the right of center," he said, adding that the coalition would organize this year around repeal of the Gramm-Rudman-Hollings budget-balancing act, opposition to the Reagan administration budget and support for a ban on nuclear-weapons tests.

The same mix of disdain for administration policies and warnings to Democrats was voiced by others, including people with whom "we have not had an historic relationship," in Jackson's words.

"We want to send [the Democratic Party] a message that we don't believe in being . . . pale imitations of Republicans," said William Winpisinger, head of the International Association of Machinists.

The union leader said his appearance at the D.C. Convention Center did not constitute endorsement of a potential Jackson presidential bid in 1988. But he praised the coalition as "the only progressive grass-roots political organization" at work nationwide.

Another non-traditional source of support on hand was Iowan Merle Hansen, president of the North American Farmers Alliance. Several busloads of farmers are expected to attend the coalition's farm breakfast today.

Jackson was joined at the news conference by District Mayor Marion Barry, California Assembly member Maxine Waters, environmentalist Barry Commoner, David Cortright, executive director of the National Committee for a Sane Nuclear Policy, and C. Delores Tucker, head of the Democratic National Committee's Black Caucus.

Two prominent black mayors who did not support Jackson for president -- W. Wilson Goode of Philadelphia and Richard Arrington of Birmingham -- are expected to participate later in the three-day convention.

"I think some of the black leaders who were afraid to make a break with tradition in 1984 wound up being embarrassed by not being where their constituents were," Waters said.

Last night, Rep. Mickey Leland (D-Tex.), who did not support Jackson in 1984, hosted a reception for him at the Rayburn Office Building. Leland chairs the Congressional Black Caucus.

The convention plans to establish procedures for chartering local chapters and endorsing candidates.

Most candidates who receive coalition support this year are likely to be Democrats. In Vermont, however, Democratic National Committee member Ellen David-Friedman, head of the state Rainbow Coalition chapter, said her chapter plans to support Bernard Sanders, self-described Socialist mayor of Burlington, in his independent gubernatorial bid.