Former House Speaker Newt Gingrich’s proposed “humane” approach to enforcing U.S. immigration laws could mean legalization for as many as 3.5 million illegal immigrants, according to a new study by the Pew Research Center.

Under the broad outlines of Gingrich’s plan, the government would not expel illegal immigrants who have lived in the U.S. for decades, raised a family, paid taxes, maintained ties to churches or community groups, and have no criminal record aside from unauthorized residency.

Thirty-five percent of the nation’s estimated 10.2 million illegal immigrants have lived in the U.S. for 15 years or more, Pew found, while 70 percent have been residents for at least a decade.  Roughly half are parents of young children.

Nearly 40 percent of Hispanics who are not citizens or legal permanent residents attend church services weekly, according to the Pew study, which analyzed data from the Labor Department, the  Census Bureau and an independent national survey of Latinos.

“I don’t see how the party that says it’s the party of the family is going to adopt an immigration policy which destroys families that have been here a quarter century,” Gingrich said last month.

“I’m prepared to take the heat for saying, let’s be humane in enforcing the law without giving them citizenship, but by finding a way to create legality so that they are not separated from their families,” he said.

Gingrich has not definitively said where he would draw the line on allowing long-term illegal immigrants to apply for legal residency, though he has suggested 25 years in the U.S. could be a marker.

Pew Hispanic Center demographer Jeffrey Passel said he is unable to estimate the number of illegal immigrants living in the U.S. for a quarter century because the existing data is not “robust.”

“A lot of the people who came in as undocumented immigrants 25 years ago are already legalized,” Passel added, referring to legalization programs in the 1980s.

Meanwhile, the share of long-term illegal immigrants as a percentage of the overall population continues to climb as flows of younger immigrants have fallen off sharply in recent years.

“Because we don’t have as many new ones coming in and the ones who have been here don’t seem to be leaving, you have this accumulation of people who have been here a long time,” he said.

Only 15 percent of the U.S. illegal immigrant population in 2010 had been in the country less than five years – down from 32 percent in 2000.

Experts attribute the decline to a down U.S. economy and enhanced border enforcement.