“Then there’s the effect of the pick on Ryan’s home state of Wisconsin. There has been a surge toward Romney since the Ryan pick, and the state is now generally considered a toss-up. On June 11, Obama was up by 4.4 points in the RCP average; as of today, that’s 1.4 points.
“In a very close race, Wisconsin’s 10 electoral votes — votes that electoral-college fanatics playing with all kinds of scenarios to get Romney to the 270 he needs to win have consistently assigned to Obama — could be decisive.
“Even more telling is what Wisconsin polls suggest about Obama’s standing nationwide. If things go badly for him in the fall, Wisconsin will have been the great harbinger, beginning in 2010.” (source)