WACO, Tex.  Playing on anxieties about national security, Senator Hillary Rodham Clinton has produced a “red phone moment” advertisement that suggests she would be better able to respond to a crisis than Senator Barack Obama.

“It’s 3 a.m. and your children are safe and asleep,” says a narrator as threatening music surges over dark black-and-white images.

There’s a world crisis and the White House phone is ringing. “Your vote will decide who answers that call,” the narrator says. “Whether it’s someone who already knows the world’s leaders, knows the military  someone tested and ready to lead in a dangerous world.”

It ends with a photo of Mrs. Clinton wearing glasses and picking up the phone.

Mr. Obama, responding to the ad during a stop in Houston, said it raised “a perfectly legitimate question.”