The ad says Romney wants to extend tax cuts for millionaires at the expense of middle-income earners.
Romney has called for an extension of tax cuts first approved by George W. Bush for all income earners. Obama wants to let the cuts expire at the end of the year for families making more than $250,000 a year.
The ad says Romney doesn't see the "heavy load" the middle class is carrying.
The new spot is airing as Democrats open their party convention in Charlotte, N.C., where they'll focus on Obama's economic plans for the middle class.
The ad is running in Colorado, Iowa, New Hampshire, Nevada, Ohio and Virginia, key battleground states.
Responding to the ad, Romney campaign spokeswoman Amanda Henneberg said Obama is the only candidate who wants to raise taxes, while Romney has a plan to lower rates, help create jobs and boost take-home pay.