In the defining moment of a testy debate, President Barack Obama lashed into Mitt Romney on Tuesday over the Republican’s criticism of his handling of a deadly attack on the U.S. diplomatic mission in Libya and sought to cast him as unfit to become commander-in-chief.Moving to regain lost ground after a weak performance in the first presidential debate, Obama fought back against his rival’s accusations that he had played down the September 11 assault by Islamist militants in Benghazi that killed the U.S. ambassador and three other Americans.Romney, who dominated the candidates’ previous encounter, appeared stunned as Obama launched a finger-wagging counterattack over an issue that has become a flashpoint with just three weeks to go in a presidential race considered too close to call.The exchange came near the end of a debate dominated mostly by arguments over the economy, jobs and taxes, considered voters’ main concerns in the November 6 election.Romney and his aides have sought to use the Benghazi incident - as well as anti-American unrest in other parts of the Arab world - to dent Obama’s national security credentials and accuse him of pursuing a failed Middle East policy.But Obama came out swinging in their second debate at Hofstra University in Hempstead, New York, accusing Romney of exploiting the Benghazi attack in an effort to score "offensive” political points.