Update: A newspaper already knew and had evidence of his affair (via e-mail) back in December.
South Carolina Governor Mark Sanford, had resigned his post as Chairman of the RGA. Sanford just admitted to having an affair with a women from Argentina. Governor Haley Barbour will most likely take over the RGA earlier then expected now. He left the press conference as someone asked if he would resign his post as Governor of South Carolina.
Senator Ensign, Jon Gosselin and now Governor Sanford. Perhaps they could start a new political party..."The Affairs Party".
Will this hurt his chance in 2012? YES. Same with Ensign. However, it's better to get this out in 2009 and have a few years to be forgiven then to get this out in the heat of the primary, or worse, the general election.