In a worst-case scenario quietly presented last month by a joint CIA-FBI agency that analyzes intelligence, al Qaeda would use weapons of mass destruction to launch multiple simultaneous attacks on the United States and overwhelm the U.S. government. It was an extreme possibility on a menu of unknowns that had less-threatening options. All 535 members of Congress continued to work as usual until this week, when Sen. Mark Dayton (D-Minn.) shut his offices, sent his staff home and cautioned people against visiting Capitol Hill. The surprising response by the freshman senator from Minnesota to the latest in a series of...