In a little more than a decade, since he won the popular vote but lost the presidential election to George W. Bush in 2000, the former vice president and Tennessee senator Al Gore has been a best-selling author; starred in an Oscar-winning documentary, “An Inconvenient Truth” (2006); won the Nobel Peace Prize, in 2007, for his environmental activism; and made a fortune some have estimated to be bigger than Mitt Romney’s, thanks in part to the recent sale of his stake in the Current TV network to Al Jazeera.