Between 2000 and 2010, the illegal immigrant population of Texas increased by 60 percent, while California and Florida had “no increase.”

During the Oct. 18, 2011, Republican presidential debate in Las Vegas, former Massachusetts Gov. Mitt Romney sparred with one of his rivals, Texas Gov. Rick Perry, over immigration.

Over the last 10 years, Romney said, California and Florida have had “no increase in illegal immigration. Texas has had 60 percent increase in illegal immigrants…. If there’s someone who has a record as governor with regards to illegal immigration that doesn’t stand up to muster, it’s you, not me.”

We’ll start our analysis by noting that counting illegal immigrants is a notoriously tricky task. The U.S. Census Bureau doesn’t ask for the legal status of the people it interviews, so independent groups need to approximate the numbers of foreign-born individuals who are believed to be unauthorized, based on factors such as gender, age, country of origin and year of entry into the U.S.

According to experts we contacted, there are two sources that produce credible estimates: the Department of Homeland Security’s Office of Immigration Statistics and the Pew Hispanic Center. Both of these organizations have produced numbers that are broadly similar to Romney’s claim.

First, the numbers for Texas. The DHS estimated that Texas had 1,090,000 unauthorized immigrants in 2000, rising to an estimated 1,770,000 by 2010 — an increase of 680,000, or about 62 percent, over the decade.

Meanwhile, Pew estimated the numbers to be 1,100,000 in 2000 and 1,650,000 in 2010 (the midpoint of a range between 1,450,000 to 1,850,000). That’s an increase of 550,000, or exactly 50 percent.

The difference between 50 percent and 62 percent is “well within the margin of error of any of these estimates,” said Jeffrey S. Passel, a senior demographer at Pew who helped assemble the estimate. So the 60 percent increase Romney cited is broadly supported by the best estimates available.

What about the numbers for California and Florida over the same period? According to the DHS figures, California had a 2 percent increase, while Florida had a 5 percent decrease. So Romney’s slightly off when he claims that California had “no increase,” but he has a point that both states were broadly stable over the period.

Meanwhile, when rating promises of these sort, we always try to look not only at whether the facts are accurate, but also whether the public official being credited or blamed for the statistic has significant authority over the issue.

In the case of immigration, governors have limited influence.

“There are certain trends with respect to illegal immigration that are beyond the ability of any single state policymaker to counteract,” said Michelle Mittelstadt, director of communications for the Migration Policy Institute, which studies immigration issues. In addition, she said, “border enforcement is almost entirely in the federal realm, and only federal policymakers can set the parameters for who has legal status in this country.”

Still, Mittelstadt added, some states in recent years “have taken on a greater role in immigration enforcement and policymaking, and state and local executives play a significant role in determining how welcoming or hostile their jurisdictions are perceived to unauthorized immigrants.” For instance, she said that under Perry, Texas has “asserted a growing role for itself at the border.”

Meanwhile, Ira Mehlman– the media director for the Federation for American Immigration Reform, a group that seeks to stop illegal immigration — added that while Perry has focused on border enforcement in Texas, he has not gone as far as, say, Arizona in “removing incentives to come [il legally],” including access to jobs and benefits — “the most important factors in controlling illegal immigration.”

Our ruling

Romney is essentially correct on the numbers, but his strong implication that Perry is responsible is a significant stretch. On balance, we rate the claim Half True.