WASHINGTON (AP) — Sen. John Kerry's speech to the Democratic National Convention Thursday night will be an audition of sorts for secretary of state if President Barack Obama is re-elected.

The current job-holder, Hillary Rodham Clinton, has indicated that she won't return after four years of punishing high-stakes diplomacy. She's traveled to 110 countries and covered more than 900,000 miles. Her departure will leave a critical job wide open. Some possible candidates are Susan Rice, the U.S. ambassador to the United Nations, and Kerry, chairman of the Senate Foreign Relations Committee.

To help the administration, the Massachusetts lawmaker shepherded a new arms control treaty with Russia to Senate ratification over conservative opposition in December 2010. He also has served as an unofficial negotiator with Afghanistan and Pakistan.