Perennial candidate and consumer guru, Ralph Nader, has decided to run for president again as an independent. Since he is generally reviled by mainstream Democrats for "giving" G.W. Bush the presidency in 2000, he will most likely not pull many votes from the Democratic candidate and tip the scales for the Republicans this year. However, his presence will have an effect on the election -- it will keep the Democrats from moving to the center when it comes time for general election maneuvering.
In other words, if Obama or Hillary start embracing free trade or get lax on corporation-bashing, Ralph will be there to let his constituency know. As long as a viable "protest vote" is available for the unrepresented left (i.e., real socialists, eco-warriors, anti-Americans), the Democrats will have to be careful not to be too mainstream.