With counting still not concluded in the southeastern state, Mr Obama secured 49.92pc of Florida's vote against 49.22pc for Mr Romney, according to the latest figures.
If confirmed, his Florida win would give him a 332-206 Electoral College victory.
The loss would mean fresh humiliation for Mr Romney, whose advisers boasted that it was "baked" for the Republican.
He became the first Republican in half a century to lose among Florida's Cuban-Americans, the party's most faithful Hispanic supporters.
The rout led to calls for reform. Condoleezza Rice, the former Secretary of State, warned Republicans were "losing important segments" of the US. "When you send mixed messages, sometimes people hear only one side of that," she said of the party's stances on immigration and women's rights.
It was also revealed that Romney's computer system for tracking turnout kept crashing, leaving strategists clueless.
ORCA was supposed to allow volunteers to use their mobile phones to check off supporters as they voted. However, the system collapsed.
One volunteer, John Ekdahl said: "30,000-plus of the most fired-up volunteers were wandering around confused and frustrated when they could have been doing anything else to help." (© Daily Telegraph, London)