President Barack Obama's reelection campaign is once again attacking Mitt Romney over the issue of taxes, releasing a new ad on Thursday highlighting how the presumptive Republican nominee would raise taxes on middle class Americans while boosting the incomes of the nation's top earners.The ad, titled "Stretch," takes a personal dig at Romney's own tax history, informing the viewer that Romney earned $20 million in 2010 but paid a rate of 14 percent -- "probably less than you."The commercial then pivots to an independent study released by the Tax Policy Center on Wednesday, which concluded that Romney's tax plan would primarily help the wealthiest few.