A new poll of four critical swing states by Purple Strategies, taken after Mitt Romney selected Paul Ryan as his running mate, shows Romney leading in three critical swing states -- Virginia, Ohio, Florida. Romney trails Obama in the fourth -- Colorado -- that Purple Strategies polled.

In Virginia and Ohio, Romney has gained ground since last month. In Virginia, a state that may determine the 2012 election, Romney leads Obama by 3 points (48% to 45%). Last month, Romney trailed Obama by two points.

In Ohio, Romney leads Obama by two points (46% to 44%). Last month, Obama led by 3 points, 48% to 45%.  

In Colorado, Obama leads 49% to 46%, and that is a 2-point improvement from last month. 

In Florida, Romney leads Obama, 48% to 47%. Last month, Obama trailed by 3 points. 

Voters in these states think Romney and Ryan are more likely to “bring real change to Washington” by 6 points and that margin, Purple Strategies notes, is 17 points in Romney and Ryan’s favor among independents. 

Romney and Ryan face a disadvantage on Medicare, though, the poll found. Voters in these four states, taken together, favored Obama-Biden over Romney-Ryan on Medicare by eight points (48%-40%). However, Romney-Ryan did best on the Medicare question in Florida, which defies what the conventional wisdom has been. 

No candidate is above 50 percent in favorability, but Ryan scores the highest (45% to 39%) among the four. Also, Ryan’s selection has helped Romney’s favorable numbers. Among the voters surveyed, 45% viewed Romney favorably while 48% viewed him unfavorably. However, Romney’s favorability rating was a net negative 8 points last month, so Romney's favorables have improved by 5 points, which is consistent with the gains the newly-formed ticket has seen in Virginia and Ohio.