Associated Press: As BP labored for a second day Thursday to choke off the leak at the bottom of the Gulf of Mexico, dire new government estimates showed the disaster has easily eclipsed the Exxon Valdez as the biggest oil spill in U.S. history. After an 18-hour delay to assess its efforts and bring in more materials, BP resumed pumping heavy drilling mud into the blown-out well 5,000 feet underwater. Officials said it could be late Friday or the weekend before the company knows if the procedure known …