While there is a small and vocal cadre of folks who are enthusiastic about Al Gore, it bears mentioning that Gore has managed to choose one of the worst possible issues to indelibly link with his name if he wants to make a serious run at the Presidency: the environment.
While unions are declining in significance as a political force in the Democratic Party, they still comprise the bulk of the 'foot soldiers' that make for effective election campaigns. Winning without the support of the unions as a Democrat is roughly equivalent to winning without the support of the evangelicals for the Republicans. And Gore-style environmentalism is a very tough sell for the unionized laborers who stand to lose good jobs if he pushes his issue too hard.
Also keep in mind that the only people substantially interested in the Nobel Peace Prize are likely to have already chosen sides - those who think the Peace Prize is a sharply diminished honor are also unlikely to vote for Gore, those who think the Peace Prize is meaningful are already likely Gore enthusiasts. For most of the population, the winner of the Peace Prize has no more impact on their consciousness than the winner of the prize in Chemistry - arguably his Oscar is more important to the majority of the population.