(Image Credit: Chris Kleponis/AFP/Getty Images; Paul J. Richards/AFP/Getty Images)

Given all the talk about the nation's problems with illegal immigration, highlighted by the Supreme Court debate over Arizona's controversial new law this week, it may come as a surprise to learn that for the first time in decades the number of Mexican immigrants coming to this country has dropped, according to a new Pew Hispanic Center study of government data from both countries.

After years and years when millions of Mexicans illegally entered the United States looking for work, the trend has now been reversed, as many undocumented workers return to Mexico, unable to find work in the U.S..

In 2007, nearly 7 million illegal immigrants from Mexico were living in the U.S., but that number has fallen to around 6.1 million, the study found.

The news comes as the Supreme Court justices prepare to hear arguments Wednesday on Arizona's strict new law requiring immigrants to carry their registration documents at all times. The law also requires police officers to question anyone believed to be in the country illegally.

Passed by the state's Republican Governor Jan Brewer, the law has become a key issue in this year's presidential campaign.

Mitt Romney, the presumptive GOP nominee, has praised the law as "a model" for the rest of the country; and he was endorsed by the bill's author Kris Kobach.

President Barack Obama, meanwhile, has challenged the Arizona law and his administration won a victory in the lower courts to prevent four of its more controversial provisions from taking effect. Now the Supreme Court will review the case.

However, neither Obama nor Romney appears to be in any hurry to weigh in on the decline in immigration. Both campaigns declined to comment. That may be because both sides are aware of the importance of the Latino vote in this year's election; Latinos are the nation's fastest-growing voting bloc - and both candidates have records that raise questions about their commitments to Latinos.

In his successful 2008 campaign, Obama promised to enact comprehensive immigration reform. But once in office - and even though Democrats controlled both houses of Congress - his administration failed to do so, focusing instead on health care reform.

Even the Dream Act, a scaled-down immigration measure that would provide a path to citizenship for some children of illegal immigrants who attend college or serve in the military, was defeated in the Senate.  Republicans widely opposed to the bill and five Democrats also voted to block it.

Not only do Obama's failed immigration promises put his support among Latinos at risk, but the fact that a weak economy has been the main cause of the recent drop in immigration is not something that the Obama campaign will be in any rush to highlight.

Throughout the Republican primary season, Romney has compiled his own record with Latinos that could cause him problems come fall. In addition to celebrating Kobach's endorsement of his candidacy, and complimenting Arizona's new law, the former Massachusetts governor said he would veto the Dream Act.  He also outlined an immigration policy based on the notion of "self-deportation."

In recent months, Democrats have repeatedly dubbed Romney "the most extreme presidential candidate ever" on the issue of immigration. Romney's "self-deportation" policy has even been mocked by members of his own party.

Recent polls indicate that Obama is favored among Latinos. A late January poll conducted by Latino Decisions for ABC News and Univision found that 67 percent of Latinos would back Obama, compared to only 25 percent for Romney. And as many as 41 percent of Latinos nationwide said they had a somewhat unfavorable or very unfavorable view of Romney.

In an interview earlier this month, Los Angeles Mayor Antonio Villaraigosa predicted that Obama will outperform himself among Latinos, winning even more than the two-thirds of the Latino vote he earned four years ago in his match-up against Republican nominee John McCain.

"I expect that the president is going to get an even higher percentage in 2012," Villaraigosa predicted. "We're not taking anybody for granted, but I think there are prospects of getting an overwhelming vote among Latinos for the president."

I think in that primary we heard things that surprised the vast majority of us and were offensive to many of us," he said, calling it the worst rhetoric in the past 40 years.

" I don't think that Mitt Romney has any credibility on this issue. I just don't," he said. "Not when you argue for the self-deportation of 11 million people. Not when you think that the Dream Act is a handout. Not when you didn't say a word in the course of those debates some of the most anti-immigrant and racially offensive and ethnically offensive statements were made. Never once did he say, 'I take umbrage with that.'"

Romney has recently gone on the offensive on immigration, attacking Obama for his failed campaign promises. At a rally earlier this month in Milwaukee, Romney said immigration reform would be "a priority" for him as president, something he said it has not been for Obama.

"This has always been a priority for the president he chooses to do nothing about," Romney said. "Let the immigrant community not forget that while he uses this as a political weapon, he has not taken responsibility for fixing the problems we have."

"This will be a priority of mine if I become president to make sure we finally reform our immigration laws step by step, secure the border, improve our legal immigration system," Romney said, "so we can keep people here and welcome people here who will make America a stronger nation."

While both sides have so far avoided discussing the drop in immigration from Mexico, Villaraigosa, during his interview with ABC News and Univision,  pointed to the recent drop in immigration as evidence of Obama's efforts to address the issue.

"The last time I looked, we've secured our borders to the point where we have a net migration people from the U.S. to Mexico, not the other way around. A net minus," he said, neglecting to mention the economic issues that are contributing to decreasing immigration from Mexico.

But with the Supreme Court poised to focus on immigration this week - and the Obama vs. Romney battle heating up on the campaign trail - expect to hear a lot more about the issue in the coming months. It is one that is not going away anytime soon.

Matthew Jaffe is covering the 2012 campaign for ABC News and Univision.