Despite a much-heralded reinforcement of U.S. border patrols, tradition and sharpened economic necessity are sending an increasing number of ambitious Mexicans to the United States in search of work.

Shrinkage of the troubled Mexican economy, estimated at about 4 percent this year, has made it likely that the numbers of Mexicans trying to sneak across the border will continue to increase in the months ahead despite the stepped-up U.S. border guard, according to U.S. and Mexican analysts.

Another factor pushing Mexicans northward, according to villagers here, is the Mexican peso's steady decline against the U.S. dollar, meaning that money sent back home buys more and makes the trip across the border increasingly worth the risk.

"The number is going up all the time," said the Rev. Jose Flores, the Roman Catholic parish priest in this village of 5,000 inhabitants in west-central Mexico's Michoacan State. "It is a chain."

Emigration has been a tradition around Tarimbaro ever since braceros traveled to California in the 1940s to work in the fruit and vegetable fields under contracts approved by the U.S. and Mexican governments. Isidoro Reyes, now retired at 73, said he built his house here from money earned picking fruit back then in orchards from California to Washington State.

"And for that, thanks to the United States," he declared.

The tradition has become so ingrained that nearly everyone encountered on a stroll around the Tarimbaro town square has a relative working in the United States. Mayor Adolfo Martinez said many poor farmers and their sons in this area have acquired the idea that easy fortune awaits in the United States. He qualified the idea as an "illusion," but recalled that his own father was among those who traveled north to work 20 years ago.

"The older ones give the younger ones the idea to leave," said Gilberto Torres, 28, a state government clerk whose brother cooks in a San Antonio restaurant. "They say it is better, and that with the pesos there that they can send back, their families will have a lot of pesos here."

Dr. Marta Gomez, a physician practicing here, said the idea that heading north brings wealth is so broadly accepted that she recommended a patient to a more expensive private hospital because he said he had just returned from the United States. As it turned out, she said with a smile, the patient could not pay his bills and should have gone to a public hospital.

Agustin Martinez, a laborer from the San Jeronimo area in neighboring Guanajuato State, provided a simple explanation of why the "illusion" persists. Martinez said he has worked a number of times fixing fences on ranches near Laredo, Tex., for $12 a day. Although low wages by U.S. standards, Martinez compared it to the $2 a day he has been earning this month near home doing similar work. "You just can't make it here," he said.

Martinez said he tried to sneak across the border again two months ago but the Rio Bravo, which forms the border at Laredo, was flooding, making it impossible to wade across. His plan now is to work around San Jeronimo until he hears from friends that the river is back down, and then try again, Martinez said.

Martinez, 34, said he has been across the border for three- or four-month stretches a number of times, without being caught by U.S. immigration authorities. He has sent home money orders to support his four children, left in the care of his 64-year-old mother.

Although his wife left him, Martinez said, he used the money from his last stay to build a little cinderblock house for himself and his children. His mother's house -- built on land bought by his father with money from another generation's work in the United States -- was too small to hold his children and those of his brothers, who he said also sneaked north for work.

"At least when they go up there, you have a little hope," his mother told a visitor.

In search of such hope, the U.S. Immigration and Naturalization Service has estimated, up to 1.8 million illegal immigrants will cross into the United States from Mexico this year, a 50 percent increase over 1985.

Increasingly in recent months, young Mexicans are deciding to sneak into the United States for long periods, or for good, according to U.S. authorities and local residents. These include people with schooling and training who have become discouraged about finding jobs in their own country.

Discouragement has grown in part because of austerity measures enacted by President Miguel de la Madrid's government to meet loan requirements set by the International Monetary Fund and Mexico's creditor banks, most of which are American. A new round of talks has reached a critical phase, with the possibility of new austerity measures in the offing.

In Mexico City, Sen. Guadalupe Gomez recently submitted a report to the Mexican Senate and the U.S.-Mexican Interparliamentary Meeting that found that illegal aliens crossing the border "increasingly have a higher level of schooling and better qualifications as workers."

Father Flores, speaking of this trend, recalled a young couple who got married in his crumbling church recently "and left for the United States the next day."

Antonia Hernandez, assistant principal at the local secondary school, emphasized that student advisers seek to discourage youths from thoughts of working in the United States. The alternatives, she said in an interview, are traveling to Mexico City or to Morelia, the nearby state capital. Sociologists have found that many youths do try to find work in Mexican cities first, then head for the border when they fail.

The division of farmland into small plots inhabited by families with eight or 10 children makes it almost inevitable that youths have to go somewhere when they reach their late teens, Flores explained. Only two factories operate near Tirambaro, each employing about 15 people, Torres said.