Writing at the Daily Beast, Michael Tomasky argues that Obama's grip on the Electoral College is such that Romney, to win, "has to draw an inside straight like you've never seen in a movie." He wrote that on August 4. A few days later, Romney picked Ryan as his running mate, which I think indicates his campaign agrees with Tomasky's analysis. Either something has to change--and Ryan is more apt to change the dynamic than, say, Portman or Pawlenty--or else the Mittster has to carry a lot of states in which he currently appears to be trailing.
Look at the four states Romney toured with Ryan immediately after adding him to the ticket: Virginia, North Carolina, Florida, and Ohio. George W Bush won all four in both 2000--I'm not going to mention what happened in Florida that year--and 2004. It's another tacit admission that Romney is behind and flailing. He needs to win all of them and he's ahead, barely, only in North Carolina. If he were in as strong a position as he says he is, he'd have picked Pawlenty, and his travel itenerary would feature Pennsylvania, Michigan, Colorado, New Mexico, and Nevada.