The campaign to create a national park dedicated to the once-top-secret Manhattan Project is moving through Congress, but supporters aren’t ready to declare victory just yet.

“It is by no means a fait accompli,” says Nancy Tinker, senior field officer for the National Trust for Historic Preservation.

Still it’s the closest the park has come yet to being a done deal.

The U.S. House approved in June the $552.1 billion defense authorization bill, which included funds to establish the Manhattan Project National Historical Park, which would include sites in Oak Ridge, Los Alamos, N.M., and Hanford, Wash....