Migrants jump out of a tractor trailer as Mexican federal police watch at police headquarters in Tuxtla Gutierrez, Mexico, Sunday June 12, 2011. Each year, thousands of migrants cross southern border on their way to the U.S. (AP) (Alejandro Estrada/AP)

A four-decade tidal wave of Mexican immigration to the United States has receded, causing a historic shift in migration patterns as more Mexicans appear to be leaving the United States for Mexico than the other way around, according to a report from the Pew Hispanic Center.

It looks to be the first reversal in the trend since the Depression, and experts say that a declining Mexican birthrate and other factors may make it permanent.

“I think the massive boom in Mexican immigration is over and I don’t think it will ever return to the numbers we saw in the 1990s and 2000s,” said Douglas Massey, a professor of sociology and public affairs at Princeton University and co-director of the Mexican Migration Project, which has been gathering data on the subject for 30 years.

Nearly 1.4 million Mexicans moved from the United States to Mexico between 2005 and 2010, double the number who did so a decade earlier. The number of Mexicans who moved to the United States during that period fell to less than half of the 3 million who came between 1995 and 2000.

The trend could have major political consequences, underscoring the delicate dance by the Republican and Democratic parties as they struggle with immigration policies and court the increasingly important Latino vote.

Illegal immigration has emerged as one of the most emotional political issues in the country — one that dominated much of the Republican presidential contest and has proven complicated for President Obama.

Mitt Romney has courted conservatives with aggressive anti-illegal immigration rhetoric. But the GOP presidential hopeful has said in recent days that he wants to build ties with Hispanics, many of whom have chafed at his statements, and the new immigration trends could offer him a chance to soften his stance.

Obama has been criticized by immigrant advocates for stepped-up deportation policies that analysts have said were partly responsible for the decreasing flow of Mexicans into the United States. The trend could offer the president a political silver lining: the chance to take credit for a policy success that, his aides have said in the past, should persuade Republicans to embrace a broad immigration overhaul plan.

According to the report, the Mexican-born population, which had been increasing since 1970, peaked at 12.6 million in 2007 and has dropped to 12 million since then.

The reversal appears to be a result of tightened border controls, a weak U.S. job and housing construction market, a rise in deportations and a decline in Mexican birthrates, said the study, which used U.S. and Mexican census figures and Mexican government surveys. Arrests of illegal immigrants trying to enter the United States have also dropped precipitously in recent years.

Whether the reversal is temporary or permanent, it could have significant implications for the United States. Many Mexican immigrants work in agriculture and construction.

One in 10 people born in Mexico live in the United States, and more than half entered illegally. Most live in California and Texas; about 120,000 live in the Washington region.

Immigration from Mexico has plummeted (The Washington Post/Source: Pew Research Center)

The report does not specify how many of those who moved to Mexico had been in the United States illegally. But the statistics imply that many of them had been: The number of undocumented Mexicans here dropped from 7 million in 2007 to 6.1 million in 2011, while the number of those here legally increased slightly, from 5.6 million in 2007 to 5.8 million in 2011.

“The diminished flow appears largely to be a drop in unauthorized immigrants,” said Jeffrey Passel, a senior demographer at Pew and a co-author of the report. He said an estimated 5 to 35 percent of the recent returnees to Mexico were deported.

Although most Mexican deportees say they will try to return, their numbers are shrinking, too, the study said: According to a Mexican government survey, 20 percent of deportees in 2010 said they would not return to the United States, compared with 7 percent in 2005.

Half of those returning to Mexico took their entire families, including more than 100,000 U.S.-born children of Mexican immigrants. Children born in the United States to Mexican nationals are citizens of both countries.

The drop comes at a time when overall immigration to the United States continues to grow, and reflects several factors specific to Mexico, including a relatively strong economy and a sharply diminished birthrate.

In 1960, a typical Mexican woman was expected to have more than seven children, but by 2009 that number had dropped to just over two — a decline that presages a sharp reduction in the number of young workers seeking to come to the United States.

As immigration reform continues to be a divisive political issue, experts on both sides of the debate disagreed over the implications of the report.

Those advocating for a path to legalization for immigrants here illegally said the plummeting of Mexican immigration should allow for thoughtful reform to take place without the pressure of trying to stem the flow across the border.

“It gives us the space to figure out how do we fix the legal immigration system so when the economy bounces back, how do we respond?” said Clarissa Martinez, director of immigration and civic engagement at the National Council of La Raza, a Latino advocacy organization.

Others warned that the trend could reverse itself if the U.S. economy improves or the Mexican economy falters. “The idea that this respite means the problem is over is just jumping the gun,” said Mark Krikorian, executive director of the Center for Immigration Studies, which advocates for stricter immigration controls. “It’s wishful thinking by people who just want amnesty.”

But the era of entire villages moving from Mexico to the United States may be over, said Randy Capps, a senior policy analyst and demographer at the Migration Policy Institute.

Instead, he said, the current reversal may be similar to the reduced flow from Germany and Ireland a century ago. He predicted a negative feedback loop as fewer potential immigrants have connections to the United States.

“If this goes on for much longer, it’s going to take a lot to reverse it,” Capps said. “A lot of migration is based on networks — people who know people who know about the environment they’re going to be moving into. When the jobs disappear and the people you know aren’t there anymore, this channel of communication either dries up or it becomes so negative that it just changes everybody’s mind.”

Gustavo Velasquez, 38, who came from Oaxaca, Mexico, 12 years ago and serves as the director of the D.C. Office on Human Rights, said that the scarcity of U.S. jobs is causing more Mexicans to think twice about moving.

It is better to be unemployed in Mexico than to be unemployed in the United States, he said, because most migrant workers leave their families in Mexico. “They miss the warmth of being in a welcoming community,” he said, adding that with tougher border control and more deportations, Mexicans would rather be in a “precarious situation than in a situation of fear.”

Staff writers Stefanie Dazio, Carol Morello and Peter Wallsten contributed to this report.