WASHINGTON -- Arrests of illegal immigrants along the U.S. border with Mexico are at the lowest level since the Nixon administration, indicating that fewer people are attempting to cross the border to live or work in the United States. The development could change the debate on illegal immigration from securing the border to handling the people who are already here.

It's the sixth straight year apprehensions have dropped.

"Increasingly the problem is the 11 million people (in the country illegally), rather than the border itself," said Demetrios Papademetriou, president of the Migration Policy Institute, a non-partisan research organization.

In the fiscal year that ended Sept. 30, Border Patrol arrested 327,577 people trying to cross the southern U.S. border. Meanwhile, Immigration and Customs Enforcement officials deported a record 396,906 people over the same period. That marks the first time in decades that formal removals from the U.S. outpaced arrests at the border.

The number of arrests of people trying to sneak across the border has been steadily declining since 2006, after an all-time high of more than 1.6 million apprehensions in 2000. During those 10 years, more immigrants have become settled residents of the U.S.

According to the Pew Hispanic Center, nearly two-thirds of the country's estimated 10.2 million adult illegal immigrants have been living in the United States for at least 10 years. A decade ago, fewer than half had been in the U.S. that long.

"This is all part of a larger picture that we're not seeing very many new undocumented immigrants coming in, so the share of new undocumented immigrants is smaller," said Jeffrey Passel, senior demographer at the Pew Hispanic Center. "A lot of people are staying. They've put down roots. There clearly hasn't been a large scale departure of people who have been here a while."

But politicians are still fighting over who is best equipped to secure the border.

Attempts to pass immigration reform legislation have repeatedly failed, with Republicans saying they won't support any bill that provides a path to legalization for illegal immigrants who are here and won't consider other reforms until the border is secure.

Some GOP presidential candidates, including former House Speaker Newt Gingrich, have signed a pledge to build a fence along the length of the southern border -- there is already more than 600 miles of towering steel fencing in place. Texas Gov. Rick Perry, who does not support a border fence, has proposed adding to the 1,200 National Guard troops currently stationed along the border in a support role. Perry and Gingrich have both spoken of the need for "humanity" in dealing with illegal immigrants who are already here, and were both criticized by conservative Republicans.

Homeland Security Secretary Janet Napolitano has said the border is more secure than ever before "and it is clear from every measure we currently have that this approach is working."

But the weak U.S. economy and tough new immigration laws in states such as Alabama and Arizona likely play as much of a role in the drop in illegal crossings as increased security efforts, said Doris Meissner, former head of the Immigration and Naturalization Service and a senior fellow at the Migration Policy Institute.

"There is no single thing we can point to," Meissner said. "I think it's perfectly legitimate to say that border security is working. But it is not legitimate to say they are entirely responsible. Obviously it's a combination of the economy and enforcement."

It is likely that border security will remain a divisive political issue, particularly in the 2012 presidential election, Papademetriou and others said.

"During an election year where immigration threatens to become a key issue, I suspect that Republicans would want to argue strongly that the border is out of control," Papademetriou said.

Republican strategist Danny Diaz said the debate can't shift to the question of how to handle illegal immigrants living in the U.S. until Republicans are convinced the border is secure.

"I'm sure there is some credit that's due to (enhanced security), but I don't think anyone would argue that the border is secure," Diaz said. "That's just not true. I believe if the economy was improved, those numbers would go up."

Simon Rosenberg, head of the liberal-leaning NDN advocacy group, said while the debate should shift away from the border it isn't likely to because Republican calls for securing the border have repeatedly been popular with party voters.

"They are still heavily invested in it because it worked for them," Rosenberg said. "The Republican argument at the border is more ideological than fact based."

And because it is unclear whether the lower rates of illegal immigration will continue, Democrats aren't likely to proclaim the border secure just yet, Papademetriou said.

"We still have not had a real test of how good our border controls are and the reason for that is that the U.S. economy has not really picked up to the point where people on the other side ... feel that there are great opportunities for work in the United States," Papademetriou said. "This next year will be a critical year."