Moderator Bob Schieffer asked the candidates if they would declare that they would consider an attack on Israel as an attack on the U.S., essentially putting the country under America’s nuclear umbrella. He cited the U.S. mutual defense pact with Japan as an example.
One bit of important context is that the U.S. declared this policy with Japan in the wake of World War II specifically so that the country would not need to rebuild the enormous military that it had used with such disastrous results in the 1930s and 1940s. The U.S. policy also made it unnecessary for Japan to build nuclear weapons.
Israel already has a strong military and already has nuclear weapons, so extending the U.S. nuclear umbrella to Israel would not be fully comparable to the U.S.-Japan Mutual Defense Treaty.