The idea of a grass-roots coalition of blacks and Hispanics, two bourgeoning political forces, gained qualified support here today after a test run at a four-day meeting of the nation's largest Hispanic civil rights organization.

Delegates to the 54th annual meeting of the League of United Latin American Citizens (LULAC) repeatedly likened their concerns to those of blacks, minimized differences between the two groups and responded enthusiastically to the Rev. Jesse L. Jackson's call for a political "rainbow revolution" in the 1984 elections.

Five of the six announced Democratic candidates for president, all of whom are white, lauded the idea here of a black-brown coalition in some form, and when the convention concluded today, the delegates endorsed efforts by the organization to build such a coalition.

The reception accorded one of the candidates, former vice president Walter F. Mondale, was just as thunderous as that for Jackson, who has been advocating a black presidential candidacy based in part on alleged indifference of white candidates to key issues affecting blacks and Hispanics.

Jackson, expected to make a decision on running for president by September, has finished a distant third to Mondale and Sen. John Glenn (D-Ohio), but slightly ahead of four others in early polls.

"Hispanics are going to support a viable candidate," said delegate Raul Vasquez, a Mexican-American lawyer from Belton, Tex. "For Rev. Jackson to get brown votes, he's going to have to establish himself as a viable candidate."

Tony Bonilla, the league's outgoing president, said Hispanics might rally behind a black candidate, but only if they could help shape the platform and not relinquish their position as deal-makers at a brokered nominating convention.

"If Hispanics are going to be asked to play that role," he said, "we would ask no less of a black candidate than of a white ."

The black-brown coalition was a major thrust of some at a convention laced with efforts to win recognition for Hispanic political power. The group today elected as president a Mexican American, Mario G. Obledo, 50, a Sacramento lawyer and former California secretary of health and welfare.

Earlier he had emphasized the need for greater political involvement as a key to solution of social and economic problems facing Hispanics.

Bonilla and other Hispanic leaders have campaigned in Hispanic communities on behalf of high-profile black candidates such as Chicago Mayor Harold Washington, who was boosted into office by a black-brown coalition, and have exchanged visits, advice and awards with black civil-rights groups in several cities.

The league showcased its "Project Participar," designed to provide computerized technical assistance to voter-education efforts and to cultivate potential voting strength among youngsters and Hispanics who have not yet sought U.S. citizenship.

William Velasquez, executive of the San Antonio-based Southwest Voter Registration Education Project, promised funds for any league-sponsored voter-registration project proposed to him.

Of the estimated 15 million Hispanics in the nation, 30 percent are legal or illegal aliens, and 40 percent are too young to vote, although they are coming of voting age at a rate of 100,000 a year.

In 1980, 3.4 million Hispanics were registered voters, Velasquez said. But, as of 1982, Hispanic registration stood at only 35 percent of those eligible, compared to 59 percent for blacks and 64 percent for whites, according to Project Participar.

Like black leaders--who have formed a Black Coalition for 1984, endorsed the idea of a black candidacy and drawn up a people's platform--Hispanics have established "Hispanic Force '84," headed by New Mexico Gov. Toney Anaya (D).

While blacks' voting strength is concentrated in the South, Hispanics are strong in the Southwest and Florida, with Hispanic mayors in Santa Fe, San Antonio, Miami and Tampa and sizable potential voting blocs in Texas and California.

Blacks and Hispanics differ on some key issues. For instance, LULAC and other Hispanic groups are strongly opposed to immigration-reform legislation that would impose sanctions on employers who knowingly hire illegal aliens, contending that it would lead to wholesale discrimination against persons who look foreign.

The NAACP, the nation's largest and oldest black civil-rights organization, supports the sanctions proposal, contending that sanctions are necessary to prevent illegal aliens from taking low-skill jobs from blacks and others.

Bonilla said many Hispanic leaders, like their black counterparts, are cool about a black presidential candidacy, because they are committed to one of the announced candidates.

Just as black political strength is most evident in local offices, Hispanics in the Southwest have traditionally been more concerned about school boards and city councils than Congress and the White House, although that may be changing slightly, Velasquez said.

For years, polls by the Southwest voter project found local issues paramount in the minds of respondents, he said. "Since February, it's unemployment," Velasquez said. "That is not something that can be resolved by going to city hall."

Seventy percent of Hispanic voters in 1980 supported the Democratic presidential candidate. But in recent polls, he said, 86 percent of those surveyed said they would support a Democrat for president.