Immigration supporters dressed as Uncle Sam and two construction workers walk through the crowds during a rally at the U.S. Capitol in April 2013. (Linda Davidson/The Washington Post)

Undocumented immigrants hold more white-collar jobs and fewer blue-collar jobs today than they did before the national recession of 2007-2009, but most remain concentrated in lower-skilled, low-paying jobs, “much more so than U.S.-born workers,” according to a report released Thursday by the Pew Research Center in the District.

The report, based on a five-year study from 2007 to 2012, found that the size of the illegal immigrant workforce has remained at 5.1 percent of all workers, even though the total number of illegal immigrants has fallen from a peak of 12.2 million in 2007 to about 11.2 million in 2012.

Overall, it described an undocumented workforce that has stabilized during a period of economic recovery and has moved up in certain semi-skilled and professional occupations, but that has remained limited by language barriers, poor education and legal status and clustered in low-level occupations.

The senior demographer for Pew, Jeffrey Passel, presented the report’s findings at a Senate hearing Thursday. He stressed that as the illegal population has fallen, its remaining workers have settled into the U.S. and their job prospects have shifted mostly in response to national economic trends.

“These are people who are embedded and participating in the economy,” Passel said after the hearing. “More than 60 percent have been here at least ten years. The changes we see in what they are doing and where they work reflect broader trends.”

Although the majority of illegal immigrants are from Mexico, the report noted that in the Washington region, the largest number are from Central America. In Virginia, Maryland and the District, they are employed mainly in service jobs in hotels, restaurants, buildings and grounds, and private homes.

Nationwide, Pew reported, the portion of unauthorized workers in professional or business jobs — including sales and office support — grew from 10 to 13 percent, while the portion in production and construction declined from 34 to 29 percent. As in the past, though, it said “a solid majority” still toil at low-wage jobs including farm labor.

Critics of illegal immigration complain that undocumented workers squeeze out American-born workers by undercutting wages. The Pew report did not address that issue, but it found that far higher percentages of illegal immigrants than U.S.-born workers now hold jobs with the least desirable conditions, such as crop picking and animal slaughter.

Sen. Thomas Carper (D-Del), the ranking Democrat at Thursday’s hearing, said that U.S. officials “need to get serious about enforcing laws against the hiring of undocumented immigrants,” but that the government must also “provide adequate ways for immigrants to work here legally when we do need help.”

In certain occupations, the contrast between undocumented and U.S.-born immigrants is especially dramatic. Only 0.5 percent of U.S-born workers are employed in farming, fishing or forestry, while 4 percent of unauthorized aliens work in those fields,

Although the portion of illegal immigrants in professional jobs remains low, the report noted that, under several policies enacted or proposed by the White House, a substantial number are obtaining temporary legal status, giving them access to better education and professional jobs.

The work performed by illegal immigrants varies among states, the report found. In 39 states and the District, the largest number work in service jobs, but in 34 states, they hold the largest share of all farming, fishing and forestry jobs.

Nationwide, unauthorized immigrants are clustered in a few occupations, notably farming, fishing and forestry (26 percent of the workforce), building and grounds (17 percent), and construction and mining (14 percent). They comprise 24 percent of all groundskeepers, 23 percent of domestic workers and 20 percent of those in clothing manufacture.

In addition, they have carved out niches in certain relatively well-paid construction trades. They hold 34 percent of all jobs in drywall installation, 27 percent in roofing and 24 percent in painting. Passel also noted that many illegal immigrants who overstayed temporary visas have higher education levels that enable them to work in office or technical jobs.