Obama is basing the number on 10 years of the cost of the tax cuts Romney has posted on his own web site.
Some fact-checkers and news professionals say this is false or mostly false, because Romney plans to offset the tax cuts by closing loop-holes and deductions for the wealthy, so they pay the same as they pay now.
1) If we only have specifics on the tax cut side, isn't it fair to talley them up and say this is how much we are talking about over 10 years? If Romney announced tomorrow that the mortgage interest deduction was on the table for everyone making more than $200K, then that would change the total, but until and unless he sets out the plan for offsetting the cuts, this seems like a fair attack. IMO, calling this a lie is like saying the Sox are down five runs, and somebody calling you a liar because the game isn't over yet. The number can change, but not if Romney doesn't do anything to change it.
2) It seems like nobody is questioning how this actually helps the economy grow. The idea Republicans have always championed with regards to tax cuts is that if you put money back into the hands of the people, they will invest that money back into the economy. But if you're keeping the total tax burden of the wealthy equal to the total at the higher marginal rate, how does that do anything at all?