A person holds onto the fence marking the border between the United States and Mexico in Ciudad Juarez. REUTERS

When it comes to immigration policy, the candidates vying for the 2012 Republican presidential nomination agree: the first imperative is to halt the flow of illegal immigration across the U.S.-Mexico border.

Immigration has proved to be a contentious issue in the Republican primary. Texas Gov. Rick Perry was lambasted for signing a bill offering in-state tuition to some undocumented students, and former House Speaker Newt Gingrich drew criticism from his right flank for suggesting that immigrants with deep roots and family ties should have a path to legalization. But throughout this discord, every candidate has invoked the need to secure the border before pursuing any other reforms.

Before suspending her candidacy, Rep. Michele Bachmann, R-Minn., proposed building a double fence that spanned every mile, every foot, every inch of the border. Perry has vowed to police the border by fortifying the U.S. Border Patrol, which already deploys a record number of agents. Former Massachusetts Gov. Mitt Romney has embraced the border-first approach, and his focus on enforcement helped him win the endorsement of Kansas Secretary of State Kris Kobach, the architect of the harsh Arizona immigration law that became a model for other states.

What I support is focusing on securing the border, and when we secure the border and have convinced the American people that we do not have a flow of illegal aliens coming into the country, then we can address what we're going to do with the 11 or 15 million that are here, Romney told the Washington Examiner.

Cross-Border Migration Rate Decreasing

But the relentless focus on the border obscures an important fact: the rate of cross-border migrations is decreasing. U.S. Border Patrol arrests have been steadily declining since a high of 1.6 million in 2000. The number of undocumented immigrants in the United States essentially held steady in 2010 after dropping to its lowest level in a decade in 2009, a reversal of a two-decade trend of growth.

Experts offer different theories for why this is the case. An anemic U.S. economy has meant the jobs that typically draw immigrants to America are harder to find, the Mexican economy is improving, and stepped up enforcement may have functioned as a deterrent. But whatever the cause, demographic researchers say the data is indisputable.

No one wants to hear it, but the flow [of illegal immigration] has already stopped, Douglas S. Massey, co-director of the Mexican Migration Project at Princeton, told The New York Times. For the first time in 60 years, the net traffic has gone to zero and is probably a little bit negative.

The new dynamic has gone unmentioned on the campaign trail, where candidates have vowed to enhance border security and denounced measures to provide benefits to undocumented immigrants as amnesty.

Political campaigns are built around how do you connect with voters in visceral ways even if those ways are sometimes lack any grounding in reality, said Ben Johnson, executive director of the American Immigration Council. Right now it's still an academic fact that undocumented immigration is down; the media is still talking about illegal immigration and the politicians are still talking about illegal immigration.

Immigration Specialists: Focus Should Shift to Undocumented Adults in U.S.

Advocates charge that politicians should shift their focus to the roughly 10.2 million undocumented immigrant adults living in the U.S. A recent study by the Pew Research Center found that nearly two thirds of those immigrants have resided in America for at least 10 years. This population is inextricably bound to a younger generation that was born in America: 54 percent of families with at least one undocumented adult also have at least one U.S. citizen child.

We're not talking about recent border crossers, we're talking about people who have been living here, have started families, have been part of the community for more than a decade, said Jacqueline Esposito, director of advocacy for the New York Immigration Coalition. So that's the crisis. I think there's a lot of misconception out there that the undocumented population is people who are recent entrants and that's simply not the case.

It was these types of families that Gingrich had in mind when he said during a November debate that, If you've been here 25 years, and you got three kids and two grandkids, you've been paying taxes and obeying the law, you belong to a local church, I don't think were gonna separate you from your family, uproot you forcefully, and kick you out.

The remark drew swift criticism. Romney warned that the prospect of such leniency would encourage illegal immigration, echoing his earlier critique that Perry's decision to offer undocumented immigrants in-state tuition erected a magnet for immigrants considering crossing the border.

Emphasizing the threat of illegal immigration resonates with voters who are anxious about a weak U.S. economy and a shortage of jobs, suggested Dowell Myers, a professor in the Price School of Public Policy at the University of Southern California who studies immigration trends.

Political candidates want to present themselves as the saviors of an electorate that feels besieged, Myers wrote in an e-mail. The border may be the least of their concerns but the Great Recession leaves a lot of people feeling worried and discouraged. Defending the border becomes a symbol of taking bold action to come to the rescue.