WASHINGTON - Former U.S. vice president Al Gore said Friday that he won't under any circumstances serve as Democratic presidential hopeful Barack Obama's running mate. Unlike the office of the presidency, there are no imposed term-limits on the country's second highest office. But Gore, who considerably expanded the role of vice president, told CBS News that he's imposing his own term limits on the job. "I have a personal term limit. Only two terms as Vice President," he said. Asked whether he might reconsider if the Democratic presidential candidate "begged," Gore said his answer would remain "no". Rumours swirled around a possible Obama-Gore ticket last month after the former vice president -- who remains a very popular figure in the Democratic Party -- gave an enthusiastic endorsement of the Illinois senator after months on the sidelines during the prolonged Democratic primaries. Obama said then he doubted Gore would be willing to serve as vice president again.