Mexican immigrants living illegally in the U.S. has dropped significantly for the first time in decades. The shift comes as many illegal workers already in the U.S. see fewer and fewer job opportunities and return to Mexico, according to the Associated Press.

Mexico accounts for nearly 60 percent of the illegal immigrants in the U.S. Roughly 6.1 million illegal immigrants were living in the U.S. last year after a peak of nearly 7 million in 2007, according to the Pew Hispanic Center study released on Monday. It is the biggest sustained drop in modern history, only surpassed in scale by losses of Mexican-born in the Great Depression, according to the Associated Press.

The drop in illegal immigrants is due largely in part to the weak economy of the United States. It has decreased the number of construction and service-sector jobs that were attracting many Mexican workers, reported the Associated Press. But heightened patrols, deportations and violence along the border also contribute to the decline as do demographic changes like Mexico's declining birth rate.

Jeffrey Passel, a senior demographer at Pew who co-wrote the analysis, said Mexican immigration may never return to its height even with the U.S. economy recovering. He cited longer-term factors such as a shrinking Mexican work force.

The Pew estimates unfold in an election year with heightened awareness on immigration, since the fast-growing Hispanic population, which now makes up about 16 percent of the U.S. population, could play a key role.

The Supreme Court is also hearing arguments on Arizona's immigration law this week. Arizona's law wants to expand the authority of state police to ask about the immigration status of anybody they stop based on the rationale that federal enforcement has largely failed.

Since Arizona's law passed in 2010, five other states – Alabama, Georgia, Indiana, South Carolina and Utah – have passed similar measures.