* Obama wins re-election to second four-year term
* Balance of power in Congress likely to stay same
By Steve Holland and John Whitesides
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 U.S. voters decided between two starkly different visions for the country.
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 U.S. 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 U.S. Congress that looked likely to maintain the same partisan makeup.