President Reagan plunged into a round of ethnic politicking today in a festive campaign foray aimed at Hispanic voters, who could play a key role in his 1984 reelection strategy.

Reagan began a political barnstorming trip across the Southwest by celebrating Cinco de Mayo, a Mexican patriotic holiday, with a tribute to Hispanics that recalled his 1980 campaign themes while addressing the severe economic problems of Mexico that have echoed across the border into south Texas.

It is said that only Los Angeles has a larger Mexican American population than San Antonio.

Reagan, speaking in a downtown plaza, promised to pay "special attention" to the economic aftershocks being felt in the wake of Mexico's peso devaluation and debt crisis.

He announced the formation of an administration working group "to investigate the situation" and make recommendations "to alleviate some of the hardship caused by economic uncertainty on the other side of the border."

Reagan, who plans to visit Mexico later this year, said the administration is "trying to do everything we can to work with Mexico itself in attacking the problem . . . . This is not just your problem, it's our problem and we'll meet it together."

The White House said the working group, to be headed by a Commerce Department official not yet named, will report to Reagan in 45 days.

Today's appearance in San Antonio was the president's most outwardly political event in recent months.

While he has not decided whether to seek a second term, White House officials and party strategists said his appeal to Hispanics is part of an effort to reach key constituencies in advance of a possible reelection bid.

Stopping for lemonade and a guacamole taco after his speech today, Reagan was asked if he wasn't sounding like a presidential candidate.

"I'm not here for that today," he said.

Polls have shown increasing disenchantment among Hispanic voters with Reagan's economic program.

"It's very apparent to me that unemployment is the key thing that is motivating Chicanos to get to the polls," said Willie Velasquez of the Southwest Voter Registration Education Project.

Shouts of "Money for Jobs" wafted across the speakers' platform from a nearby street.

Reagan fared better among Hispanic voters in 1980 than any Republican presidential nominee in two decades, winning between 25 and 30 percent of their vote. Some Republican strategists think he will again have to turn to Hispanics in 1984 if he is to win Texas, particularly if black voters continue their overwhelming support for Democrats.

In his speech today, which drew only mild applause, Reagan praised San Antonio Mayor Henry Cisneros, a popular Democrat who is the first Mexican American mayor of San Antonio in more than 140 years.

He promised that the administration would "vigorously" enforce the Voting Rights Act, which has expanded voting opportunity for Hispanics and blacks.

Recalling the "fundamental values" of his 1980 campaign appeal--"God, family, work, freedom, democracy and justice--Reagan sought to identify personally with his audience.

"Having spent most of my life in California, I've almost forgotten when I didn't celebrate Cinco de Mayo," he said. The holiday commemorates the 1862 Battle of Puebla, in which 2,000 Mexicans defeated 6,000 French troops marching toward Mexico's capital.

The president boasted that he has appointed 130 Hispanics to "high-level positions," though there are no Hispanics in the Cabinet or in top White House positions.

Reagan was applauded most vigorously when he appealed for support against communism in Central America.

"I hope you agree with me that the United States can no longer remain complacent about what is happening to our neighbors and friends to the south," he said. "We can no longer find excuses for doing nothing and then hope for the best when the enemies of democracy--Cuba, the USSR, and Nicaragua--are actively working to subvert these nations."

Republican appeal to Hispanics has been weakened by unemployment, which has reached 20 percent in the Rio Grande valley.

Reagan said that "new signs of recovery are seen every day," and expressed concern for "those many people who are still waiting for the upturn to reach them."