[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2008/images/07/18/art.gorevp.gi.jpg caption="Gore has ruled out being Obama's running mate."](CNN) - Eight years in the No. 2 spot is enough for Al Gore.
The former vice president, who issued a challenge to the next President of the United States on Thursday to make the country's electricity output carbon-free in 10 years, said later that he will not under any circumstances serve as Barack Obama's running mate.
Unlike the office of the presidency, there are no imposed term-limits on the second highest office in the land. But Gore, who considerably expanded the role of vice president, said he's imposing his own term limits on the job.
“I have a personal term limit,” he told CBS News Thursday. “Only two terms as VP.”
Asked by interviewer Katie Couric whether he might reconsider if the Democratic presidential candidate "begged," Gore said his answer would remain no.
Buzz 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 sidelines during the prolonged Democratic primary.
Obama said then he doubted Gore would be willing to serve as VP again.
“Obviously Al Gore is a great public servant, he was a great vice president," he said. He may not want to be vice president again, since he's already done that for eight years, but certainly he's somebody that I'll be getting advice from as we go forward and hopefully he'll help me when I'm president."
Gore said in December that he might jump back into the political fray – but only in a bid for the top spot. A third term as vice president would mean Gore had spent more time in the office than anyone in U.S. history.