The White House escalated its response Thursday to the growing oil spill in the Gulf of Mexico, and a local official said the massive slick is projected to reach the Louisiana coast late Thursday.

In an afternoon appearance in the Rose Garden, President Barack Obama promised to use "every single available resource" of government to help with the spill, which could be one of the largest in U.S. history. The U.S. military began mobilizing for what could be a major effort to prevent environmental damage to Louisiana and other...