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Only 15 percent of nation's 10.2 million undocumented immigrants have lived in the United States for less than five years, reflecting a continued decline in new arrivals and a trend toward that population becoming more embedded in the country, according to a new report.

But Thursday's report from the Pew Hispanic Center is unlikely to temper illegal immigration as a hot topic among Republican hopefuls seeking their party's nod for president. It comes at a time when new front-runner Newt Gingrich is defending his answer at a recent GOP debate in which he argued for finding a way to allow some undocumented immigrants to stay.

Meanwhile, Texas Gov. Rick Perry, who took heat for arguing in favor of his state's policy of offering in-state tuition for undocumented immigrants, has picked up the endorsement of hard-line Maricopa (Ariz.) County Sheriff Joe Arpaio. Arpaio and recently recalled Arizona Senate President Russell Pearce led the charge on that state's controversial enforcement-only immigration law.

Matthew Burbank, a political scientist at the University of Utah, said the issue has "transcended" any data and become a litmus test for candidates among the GOP base.

"In a way, it doesn't have much to do with the number of people here or anything else," Burbank said. "It's taken on a life of its own without being closely connected to the scope of the problem."

Pew's 14-page report, relying on census data as well as the center's own researchers, attributes the drop in fresh undocumented immigrants during the past decade to a souring economy and tougher border enforcement.

It shows 35 percent of undocumented immigrants have been in the United States for more than 15 years, and 28 percent have been here 10 to 14 years.

That represents a marked contrast to 2000, when the population of undocumented immigrants here less than five years stood at 32 percent.

Pam Perlich, senior research economist at the U., pointed to the drug war as another reason for the drop in border crossings — both into the United States and back to Mexico.

"The cost of getting across is higher and it's really risky because of the militarization of the drug-cartel elements," Perlich said. "It's very dangerous to cross and so people are more likely to stay put."

The report also reveals that undocumented immigrants are more likely to have children in their household and linked that finding to a younger median age.

According to the study, 46 percent of undocumented immigrants have a minor in the house compared with 38 percent for legal immigrants. The median age of undocumented immigrants in the United States is 36.2 years old compared with 46.1 for legal immigrants.

Twitter: @davemontero