NEWBURGH, N.Y. – Sunday morning in this small, Hudson Valley city: More than 1,000 parishioners, most from Mexico, pack Spanish-language Masses at St. Patrick’s Catholic Church. Afterward, many families flock to El Azteca for its authentic tacos. If somebody needs a ride home, there are at least a dozen local taxi companies catering to newcomers born in the Mexican states of Puebla and Jalisco.

New residents from Mexico have, in the last four years, opened dozens of businesses that have begun to reinvigorate the ailing downtown district; they are the region’s fastest growing community.

It is the same story elsewhere in the Northeast. Like the other parts of the country before it, the region is finally starting to see the impact of Mexican migration.

New communities of Mexicans have arrived to fill farm, construction and domestic jobs, government data show. Population growth in states such as Pennsylvania, Massachusetts and Connecticut would be considerably slower if not for the newcomers, who are steadily bringing about the region’s biggest demographic shift in generations. And while the change has brought new vitality to some places, it has also created tension.

Rodolfo O. de la Garza, a sociologist at Columbia University, says it’s natural that Mexico is – and will be – the main source of Hispanic migration to the United States.

“Mexico is right there and Mexico is so big,” he said. “They’re not going to become the dominant group in the Northeast, but they’re going to be increasing important numerically.”

Sixty miles north of New York City, Newburgh has historically had a small Puerto Rican community. But these days, Mexicans drawn to farm work in area apple orchards, dairy farms and factories far outnumber Puerto Ricans, demographers say.

In 2000, the city’s 4,500 Mexicans represented half of all its Latinos; today, Mexicans are two-thirds of that group, demographers estimate.

Nationally, most Mexican-born residents have long been concentrated in California and Texas, said Jeffrey S. Passel, a demographer with the Pew Hispanic Center in Washington D.C. “If you go back 15 or 20 years, there weren’t very many Mexicans outside of core settlement areas of the Southwestern states and the Chicago area,” he said.

In the early 1990s, he said, California’s ailing economy and rising anti-immigrant sentiments pushed some Mexican immigrants into new places with abundant jobs such as North Carolina, Georgia and New York City.

As new immigrants kept arriving – peaking at more than 600,000 a year around 1999 or 2000 – many joined friends or family resettled in the new areas. Tens of thousands went straight from the Mexican state of Michoacan to meatpacking jobs in Tar Heel, N.C., and from Puebla to work in restaurants and private homes in Manhattan.

Another factor fueling the influx was the nation’s widespread real-estate boom, which drove steady growth in construction jobs. Last year, nearly one in five foreign-born Hispanics was a construction worker, and only 38 percent of the nation’s Mexican immigrants lived in California, Pew data show. That is down from 58 percent in 1990.

Small but rapidly expanding Mexican communities also are now in East Boston, Burlington, Vt., Central Falls, R.I., and Providence, said Martha Montero-Seiburth, a professor at the University of Massachusetts in Boston.

In Maine, where most immigrants pick apples or blueberries or work on dairy farms, Census estimates counted 4,419 Mexicans in 2003, up 48 percent from 2000.

Some have felt a backlash. In Farmingville, on Long Island, residents have repeatedly complained about Central American day laborers, and there have been several attacks on Latinos in recent years. In a move that some residents cheered but immigration advocates condemned, local officials recently closed down three homes crowded with migrant renters.