FREE trade with Mexico - a top priority for President Bush - could have a dramatic long-term impact on illegal immigration into the United States.A number of analysts say that in the short run, free trade will increase the number of Mexicans who enter the US unlawfully. Within 15 years, however, free trade should decrease those numbers by strengthening economic prosperity in Mexico and boosting the income of workers south of the border, experts say. Wages in Mexican factories are often around $1 an hour, or about one-tenth the pay of comparable jobs in the US. Immigration issues are being largely ignored in the trilateral trade talks among the US, Mexico, and Canada that began June 12. But analysts say the flow of people, as well as goods, will eventually have to be addressed. Millions of Mexicans and other Latin Americans have entered the US illegally across the southern border during the past decade. Most are fleeing poverty in search of good jobs. They have intensified a population boom in California and Texas. The number of illegal entrants decreased briefly after passage of the Immigration Reform and Control Act of 1986. But since May 1989, the numbers are up again. If the White House neglects the impact of trade on the movement of people, analysts worry that illegal immigration will eventually cause social and economic disruption in the US, particularly along the southern border. George Grayson, an expert on Mexico and a professor at the College of William and Mary in Williamsburg, Va., favors a free trade agreement as an aid to the problem of illegal immigration. "Without sustained economic growth in Mexico, you will see a flood of illegals turn into a tidal wave," Dr. Grayson says. "Even with growth, you will have illegals, but not as prolific." However, free trade carries risks, with the greatest dangers seen in the next 10 to 20 years. Dan Stein, executive director of the Federation of American Immigration Control, argues that without special arrangements, free trade could exacerbate population pressures from illegal migration in Texas, California, Arizona, and New Mexico. The reason: Free trade could speed the construction of thousands of new factories in Mexico along the US border. Already, hundreds of so-called "twin plants" operate there as a result of earlier trade deals with Mexico. Those twin factories have attracted millions of Mexicans to the US border area. Tijuana, once a sleepy city south of San Diego, has boomed to 1.2 million. Ciudad Juarez, near El Paso, Texas, has climbed to 1.3 million. Matamoros, near Brownsville, Texas, has risen to 600,000. The US Border Patrol says these impoverished urban areas are the primary source of illegal entrants to this country. Mr. Stein says: "We do not want Mexico to be like Canada, with everyone along the border. To exclude immigration from these trade negotiations is to remove a fundamental issue that affects the future of both countries profoundly." Stein wants several concessions from Mexico: 1. An agreement which would limit further development at the border, particularly near ports of entry like Brownsville and Chula Vista (San Diego). 2. Improvement of the infrastructure - roads, sewers, pollution controls, schools - in border communities. 3. Better facilities to monitor cross-border traffic in people and goods. "The border region is already well on its way to becoming a third-world state," with the US side also suffering from the twin threats of poverty and pollution, Stein says. Patrick Burns, a longtime analyst of immigration problems, says that even with free trade, illegal immigration will continue, but will not rise as fast. From that viewpoint, free trade makes sense, he argues, for ultimately the US "must take Mexico's people or its goods." Mr. Burns favors giving priority to Mexican products, since "we take goods from all over the world. Why not give preference to Mexico rather than Japan?" Burns says the impact of free trade on immigration should be studied, however, because importing more goods from Mexico could have unexpected results. "It could harm [industries in] some other countries, such as roses in Colombia and the carnations now coming out of Central America," he says. The Caribbean region's economy could also be damaged, he suggests, as production is shifted from countries like Haiti and Jamaica to Mexico. Stein, who has fought for years to get better control of US borders, concludes: "The American people seem blissfully unaware of the population juggernaut that lies just south of our border. It makes sense to use that labor, but it only makes sense if that labor stays there."