Seven Democratic candidates for U.S. president are debating in the historic city of Philadelphia, Pennsylvania, where front-runner Senator Hillary Clinton is facing increased criticism from her competitors.

As the televised debate began, second place and third place candidates, Senator Barack Obama and former Senator John Edwards, accused Clinton of changing her positions on past issues.

Clinton answered that she has been standing up against Republicans since her Senate career began. She defended her plans for reforming the social security retirement system and for ending the war in Iraq.

On Iran's nuclear program, Clinton said she is against a rush to war. She said Democrats must take strong action to prevent President Bush and Republicans from taking action on their own against Iran.

She recommended instead "vigorous diplomacy" and economic sanctions.

Clinton has a large lead in national polls, but a survey released Monday shows a tight race in the state of Iowa, where the first votes will be cast to begin the primary elections. The Iowa caucus is scheduled for January third. A strong showing in Iowa can give a candidate momentum in subsequent state contests.