Congressional staff members got a glimpse of some of the details in closed briefings this month, and four Democratic senators told Attorney General Michael B. Mukasey in a letter on Wednesday that they were troubled by what they heard. The plan “might permit an innocent American to be subjected to such intrusive surveillance based in part on race, ethnicity, national origin, religion, or on protected First Amendment activities,” the letter said. It was signed by Russ Feingold of Wisconsin, Richard J. Durbin of Illinois, Edward M. Kennedy of Massachusetts and Sheldon Whitehouse of Rhode Island.