He doesn’t have to refer to Bush by name. But he has to dispute that there’s still some mess he’s cleaning up. And he doesn’t even have to go back to the cause of the mortgage mess.
This is what he could say:
“There was something that needed to cleaned up, the huge bad mortgage debt owned by financial institutions. However, whatever you think of them, the bailouts (TARP etc.) did clean that up more than 3 years ago. Further, tax cuts didn’t cause trillions in worthless mortgages to be lent out - the idea is idiotic. Obama’s not cleaning anything up, he’s actually making a new and bigger mess.” Then run through the numbers like plummeting household income, continuing high first time unemployment claims, 3 million more people in poverty, the labor participation rate down to levels from 30 years ago etc. etc. Lot’s of numbers.
See how that works? But Romney will not say anything like that. He’ll give essentially a one sentence explanation (”I don’t think higher taxes will help the economy”).