WASHINGTON (Reuters) - The number of illegal immigrants in the United States ticked down slightly to 11.5 million as of January 2011, according to statistics released by the Obama administration on an issue likely to play a big role in the U.S. presidential campaign.

The U.S. Department of Homeland Security estimated the number has remained largely stable - just down from 11.6 million in 2010 - citing the high U.S. unemployment rate, improved economic conditions in Mexico and increased border enforcement.

"Mexico continued to be the leading source country of unauthorized immigration to the United States," according to the DHS report, noting that Mexicans made up 59 percent of the group, with 6.8 million believed to be in the country.

Illegal immigrants from El Salvador were a distant second, with some 660,000 believed to be in the United States.

States along the southwest border with Mexico topped the list with the most immigrants illegally in the country, with about 2.83 million in California, followed by almost 1.8 million in Texas, the DHS Office of Immigration Statistics report said.

Florida was a distant third with 740,000. The biggest percentage increase was in Georgia from 220,000 in 2000 to 440,000 in January 2011, according to the report.

DHS based its new estimate on figures collected as part of the 2010 Census.

The issue of immigration has been a hot topic on the presidential campaign trail with Mitt Romney, the leading candidate for the Republican nomination to face President Barack Obama in November's election, backing tougher measures to crack down on illegal immigrants.

Obama, a Democrat, has sought to block some tough new state laws aimed at pushing out illegal immigrants and he has urged the U.S. Congress to pass a comprehensive package of immigration reforms, but that effort has gone nowhere.

Hispanic Americans are a growing voting bloc and both candidates will be vying for their support in November.

(Reporting By Jeremy Pelofsky; Editing by Vicki Allen)