America’s illegal immigrant population grew by more than 500,000 last year and is now approaching 12 million, according to a study released yesterday that politicians from both parties are already seizing on to add momentum to their drive to change the nation’s immigration laws.

The study, released by the Pew Hispanic Center in Washington, a nonpartisan group that is a leading authority on measuring the undocumented, was released as the Senate begins considering legislation to overhaul America’s immigration laws. The report paints a detailed picture of a thriving illegal population dominated by families – including 1.8 million with children who are U.S. citizens – and highly employed, with 94% of illegal immigrant men working compared to 83% of the corresponding native-born population.

“Overall, the picture we have of this population is that, in spite of efforts to control unauthorized migration, the numbers have continued to grow,” a senior research associate at the Pew Hispanic Center and author of the report, Jeffrey Passel, said. Crossing the border, he said, is now “more expensive and more people are dying to get in, but the efforts there are clearly not stopping the migrants.”

Since the September 11,2001,terrorist attacks, President Bush has increased the number of border patrol officers by nearly 40%. These efforts, however, have not stopped the illegal flow, the report found.

In past studies Mr. Passel noted a slight drop in the numbers of new illegal immigrants in 2002 and 2003 that might have been attributed to stepped-up security measures. That blip has passed and, Mr. Passel said, a larger study shows the decline in illegal immigration was more likely tied to the economy than enforcement measures. “This is a population that is drawn by employment,” he said. “When unemployment is high in the U.S., the number of people is lower. When unemployment is lower, more immigrants come.”

A spokesman for the White House, Ken Lisaius, said the report, rather than showing a security failure on the part of the Bush administration, backs up the reason “why the president has called for immigration reform, and that’s why it remains a top priority for the administration.”

Mr. Lisaius said the president believes three aspects are crucial to stop the flood of illegal immigrants: border security, increased enforcement of immigration laws, and providing “more willing workers” through a guest-worker program. “Any reform must address those three aspects,” he said, “and must do so in a way that does not allow amnesty.”

Last year, the House of Representatives rejected Mr. Bush’s call for a comprehensive approach, instead opting for an enforcement-only immigration bill that introduces new measures, including building a wall on the southwestern border and imposing prison sentences of up to five years on anyone, including clergy members, who knowingly assists an illegal immigrant.

This morning, when the Senate Judiciary Committee will set about crafting its own legislation, it will attempt to include stepped up enforcement while also creating new avenues for legal immigration.

Senator Kennedy, a Democrat of Massachusetts, said yesterday that the findings of the Pew study underscored the need for comprehensive reform. “We must be mindful that unduly punitive measures that criminalize hard-working immigrants will only further drive them into the shadows,” Mr. Kennedy said in a statement.

Mr. Kennedy, with Senator McCain, a Republican of Arizona, introduced an immigration bill last year that is popular with New York’s foreign born because it would allow illegal immigrants in America to apply for an earned legalization program. After paying a $2,000 fine, learning English, and undergoing background checks, the immigrant would have a chance for permanent legal residency.

“Clearly, our system is broken and past efforts to strengthen enforcement haven’t worked,” Mr. Kennedy said. “The only way to fix it and break the cycle of illegality is to provide tough and fair enforcement while offering a path to citizenship.”

Since Congress’s last major immigration reform two decades ago, the illegal population has tripled to the present estimated 12 million from 4 million. While there was an initial drop after the passage of the 1986 law, which included an amnesty, a rapid increase followed in the 1990s, fueled by immigrants bringing their illegal family members through spreading networks, labor market demand, and lax enforcement of interior immigration laws.

The past decade has seen sharp growth, with about two-thirds of the illegal population in the country for 10 years or less, and 40% five years or less. Currently, Mr. Passel said, “I would say that what we’re seeing is a labor migration that is tied to employment opportunities both here and abroad. “Assuming immigration laws remain constant, he added, “there’s likely to be continued migration” at existing rates.

To estimate the population, the report analyzes data from the 2000 Census and Current Population Survey of March 2005, with other data sources that offer indications of the pace of growth of the foreign-born population, such as the number of apprehensions by the Border Patrol.

Illegal immigrants arrive in America by illegally crossing land or sea boundaries or overstaying temporary visas. The flow from Mexico, which arrives predominantly by sneaking across the southwest border, has far outgrown that of any country, numbering about 6.2 million. About 2.5 million more are from the rest of Latin America, predominantly the Central American countries of El Salvador, Guatemala, and Honduras. The remaining quarter is from nearly every other corner of the world, dominated by South and East Asia.

Mr. Passel reported that most illegal immigrants are actually working on the books, albeit under false names and with fake Social Security numbers. Illegal immigrants accounting for about 5% of an American workforce of 148 million, and 55% to 60% are paying Social Security taxes, the report found.

In some occupations, such as agriculture, dishwashers, maids, and butchers, illegal immigrants now compose a quarter or more of the workers. Still, not all workers are in manual labor or domestic work. About 10% to 15% appear to have college degrees, Mr. Passel said.

Another stereotype the statistics break, he noted, is the impression that the illegal immigrant is the single male day laborer waiting for work at a 7-11 parking lot. Most men are gainfully employed and belong to families in America, and women are 42% of the illegal immigrant adult population.

Out of a total of 6.6 million families containing illegal immigrants, a significant share is “mixed status,” where at least one of the children is born in America.

In a study released last June, Pew Hispanic Center reported that 7% of the nation’s illegal immigrants live in New York.