President Barack Obama on Tuesday won the battleground state of Wisconsin, depriving Mitt Romney of a key target that could help him win the White House, US television networks projected.

Wisconsin has not voted for a Republican president since Ronald Reagan in 1984, but Romney picked Representative Paul Ryan from the Midwestern state as his running mate and both candidates campaigned heavily there.

With losses in Wisconsin and Romney's Massachusetts, the ticket becomes the first since George McGovern and Sargent Shriver in 1972 in which both the presidential and vice presidential candidates lost their home states.