On Iran, widely backed international sanctions pushed by Obama have devastated that country's economy, Biden said. He rejected assertions that Obama failed to work closely with Israeli Prime Minister Benjamin Netanyahu and that the United States lacked commitment to a possible military strike to prevent Iran from becoming capable of developing a nuclear weapon.

"This president doesn't bluff," Biden said.

Ryan, however, insisted that Iran was closer now to having a nuclear weapon than it was four years ago, blaming the administration for allowing that to happen. He bluntly said that Tehran must not be allowed to become a nuclear power.

"This is the world's largest sponsor of terrorism," he said. "And if they get nuclear weapons, other people in the neighborhood will pursue nuclear weapons as well. We can't live with that."

Read a transcript of the debate or watch it online