Suppose the unthinkable happened, and terrorists struck New York or another big city with an atom bomb. What should people there do? The government has a surprising new message: Do not flee. Get inside any stable building and don’t come out till officials say it’s safe.

The advice is based on recent scientific analyses showing that a nuclear attack is much more survivable if you immediately shield yourself from the lethal radiation that follows a blast, a simple tactic seen as saving hundreds of thousands of lives. Even staying in a car, the studies show, would reduce casualties by more than 50 percent; hunkering down in a basement would be better by far.

But a problem for the Obama administration is how to spread the word without seeming alarmist about a subject that few politicians care to consider, let alone discuss. So officials are proceeding gingerly in a campaign to educate the public.

“We have to get past the mental block that says it’s too terrible to think about,” W. Craig Fugate, administrator of the Federal Emergency Management Agency, said in an interview. “We have to be ready to deal with it” and help people learn how to “best protect themselves.”