Mitt Romney can't fix the US economy. Neither can Barack Obama. The basic problem is that the world has gotten too good at making stuff (robotics, computers, etc.) and distributing it (globalization) and so we have a worldwide glut of labor. We can make all the stuff the world can afford to consume without needing all the available people to do it. The glut has the expected effect: wages decline.
The world will eventually reach a new equilibrium, but the period of getting there won't be pleasant. Americans are probably going to be poorer than we were during the 20th century.
What the government can do is adopt social policies that ease the pain of this readjustment. The Democrats will (on the whole) do that. Mitt Romney and the Republicans will make the situation worse by adopting policies that let the rich get richer.
Worse yet, they're trying to wire the dominance of the rich into the political system so that the rich will rule forever. Romney probably considers that to be a good thing. Ryan, an avid follower of Ayn Rand, certainly does.
The choice we face this November is an important one. This is why.