Obama's victory in the hotly contested swing state of Ohio - as projected by TV networks - put him over the top in the fight for the 270 electoral votes needed to clinch the White House and ended Romney's hopes of pulling off a string of swing-state upsets.
Obama scored narrow wins in Ohio, Wisconsin, Iowa, Pennsylvania and New Hampshire - all states that Romney had contested - while the only swing state captured by Romney was North Carolina, according to network projections.
Obama enters his second four-year term faced with a difficult task of tackling $1 trillion annual deficits, reducing a $16 trillion national debt, overhauling expensive social programs and dealing with a gridlocked U.S. Congress that looked likely to maintain the same partisan makeup.
Obama's projected victory would set the country's course for the next four years on spending, taxes, healthcare, the role of government and foreign policy challenges such as the rise of China and Iran's nuclear ambitions.
Romney made last-minute visits to Ohio and Pennsylvania on Tuesday to try to drive up turnout in those states, while Vice President Joe Biden was dispatched to Ohio. Obama remained in his hometown of Chicago.