Nikki Kahn/The Washington Post via Getty Images

When the history of the 2012 presidential campaign is written, one of the key story lines will be this: The Obama campaign went a long way toward destroying Mitt Romney before the Republican nominee got out of the starting blocks.

Consider this: During the two weeks encompassing the Democratic and Republican conventions - precisely the time when Obama starting building up a lead in the battleground states - the Obama campaign ran twice as many ads as Romney and all of the Republican groups combined, according to an analysis done by the Wesleyan Media Project. These were almost entirely negative ads run in the battleground states defining Romney as anti-auto-bailout, anti-woman and anti-middle class.

How could Democrats, with a big money disadvantage, beat Republicans on the airwaves? This was supposed to be the campaign where Mitt Romney and his Republican super PAC allies had a huge money advantage over the Democrats. After all, Republican outside groups have been awash in cash and that Romney himself has been on a fundraising juggernaut since he clinched the Republican nomination.

The primary factor is simply a strategic decision: The Obama campaign set out to define Romney early while the Romney campaign and its allies decided to wait until after the conventions to make a big advertising push. But today the Washington Post pointed to another factor that will help the Obama campaign continue to blunt the Republican money advantage: By law, television stations must give political candidates a discount (called "the lowest unit rate") on their campaign ads 60 days before a general election.

As the airwaves become saturated with ads, that means the national parties, super PACs and other outside groups can end up paying as much as six or seven times more for 30 seconds of airtime. The Post offers this example: in one Ohio market, a 30 second slot that costs the Obama campaign $125 is costing a conservative super PAC $900.

The cost disparity will continue to help the Obama campaign get more bang for its campaign buck than its Republican opponents, and even more than Romney himself. That's because while Romney's fundraising has been impressive, he raises much of it in higher dollar amounts, with much of the money going to the Republican National Committee. The RNC can run ads promoting Romney, but the airtime discount applies only to the candidates - not to the parties.