EDINBURG, TEX., DEC. 15 -- In this south Texas center of Hispanic voting, six Democratic presidential candidates tonight turned a debate into a contest of promises to channel federal aid to the Rio Grande Valley and to improve relations with Mexico.

Empowered to elect a third or more of the 198 delegates Texas will send to the Democratic National Convention -- the largest delegation of any "Super Tuesday" state -- Hispanic voters have become a focal point of the Democratic nomination fight, and their importance was reflected repeatedly here.

Former senator Gary Hart (D-Colo.), who surprised politicians and voters by reentering the campaign for the 1988 Democratic presidential nomination earlier today, did not participate in tonight's debate and his competitors did not discuss him at the request of the debate's sponsors.

Among tonight's topics:Massachusetts Gov. Michael S. Dukakis promised to take the $270 million President Reagan wants to send to the Nicaraguan contras and spend it instead in the Rio Grande Valley. Rep. Richard A. Gephardt (D-Mo.) declared that if he is elected president, "we are going to have regular summits" with the president of Mexico. In addition, he said U.S. border communities should get federal help in coping with the cost of the illegal immigration. Former Arizona governor Bruce Babbitt contended that a special program calling on companies to build "twin plants" in Mexico and the United States -- a program called Maquilladors -- should guarantee at least one job on the U.S. side of the border for every three in Mexico. Jesse L. Jackson promised that if elected president, he would target special programs to the Rio Grande Valley, including construction of a medical school and a law school, and a program of massive housing assistance. In an effort to display his commitment to farm workers, Jackson brought a stalk of freshly cut broccoli to the stage of the Pan American University auditorium where the debate was staged.

Sen. Albert Gore Jr. (D-Tenn.) decried what he said was administration policy of paying "more attention to our relationship with Romania than it does with Mexico." Gore contended that his proposals for the homeless would help the growing 'colonias' phenomenon here of rural slums springing up in areas where neither water nor sewage are available. Sen. Paul Simon (D-Ill.) contended that his controversial $8 billion jobs program would be ideally suited for this area where unemployment ranges from 16 to 32 percent. Asked by one of the panel of reporters whether the jobs he would create provide "dead-end" careers with little chance of advancement, Simon argued that the 32-hour-a-week jobs, paying a minimum wage of $464 a month, represent a significant improvement over the average family welfare payment in Texas of $169 a month.

When Simon was asked what he would do if the Soviet Union began to send large numbers of troops into Nicaragua, he said a president ought to deal "swiftly and firmly" with national threats but at the same time "the Soviets ought to be exercising self-restraint."

Dukakis was more direct in response to the question, saying that he would "step in and take action, including the use of military force, if the Soviet presence in Nicaragua became too threatening."