WASHINGTON - President Barack Obama won re-election to a second term in the White House on Tuesday, television networks projected, beating Republican challenger Mitt Romney after a long and bitter campaign.

Obama defeated Romney in a series of key swing states despite a weak economic recovery and persistent high unemployment as US voters decided between two starkly different visions for the country.

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Obama's narrow wins in Ohio, Wisconsin, Iowa, Pennsylvania and New Hampshire - all states that Romney had contested - effectively ended Romney's hopes of capturing the 270 electoral votes needed to clinch the White House.

At least 120 million American voters were expected to cast votes in the race between the Democratic incumbent and Romney after a campaign focused on how to repair the ailing US economy.

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 US Congress that looked likely to maintain the same partisan makeup.

US President Barack Obama makes a phone call to a volunteer as he visits a campaign field office in Chicago, on election day November 6, 2012. [Photo/Agencies]

Voters went to the polls in the US presidential election on Tuesday but the Electoral College, not the popular vote, actually elects the president of the United States.

There are 538 members of the Electoral College, allotted to each of the 50 states and the District of Columbia. Two hundred seventy votes are needed to win the election.