Polls closed on Tuesday in Ohio -- widely considered the most crucial state in President Barack Obama's re-election bid, with networks saying the race was too close to call.

Ohio, which has 18 votes in the US electoral college, is virtually a must-win for challenger Mitt Romney, as no Republican has won the White House without carrying the industrial Midwestern battleground state.

Polls also closed in North Carolina, where Romney had been favored but the battle was said to be very tight, and West Virginia, which networks called for the Republican challenger.