A billion dollars here, a billion dollars there, and you’ve got a record presidential election. The U.S. Supreme Court decision in Citizens United has unleashed a flood of big money in the 2012 campaign from wealthy individuals and others. The new breed of Super PACs played a major role in the Republican primary fight. Now, in the general election, they are helping to finance the most intensive early barrage of TV advertising in history — and the most negative one. For the first time, an incumbent president and his allies might be outspent by his challenger. We explore the battle for bucks in the contest between President Barack Obama and presumptive Republican nominee Mitt Romney. Who’s spending what — and whether it makes a difference.