On the Two-Party Duopoly in U.S. Politics

Over at Mostly Harmless, I just wondered aloud about the possibility of an Obama/McCain run as Independents for the Executive Office (if they end up #2 to Clinton and Romney in their parties' primaries). The question I want to pose to my co-bloggers here--and anyone else who wants to weigh in--is whether such a campaign would represent the best chance in our lifetimes to really shake up the two-party duopoly in U.S. politics. I wonder how many left Democrats like me would be torn between the following options and what we would decide:

• holding our noses at another Clinton presidency while working to increase the number and influence of liberals, progressives, and radicals at all other levels within the Democratic Party, which would mean accepting the exclusion of Obama along the lines of Nader and Lieberman;
  
  
• holding our noses at Obama's tactical alliance with McCain in hopes that it fatally wounds the Republican Party, forces the Democratic Party to redefine itself, brings new voters into the political process, redefines "centrism" and "bipartisanship" in American politics, moves the electorate and terms of debate in the U.S. a little bit leftward in the short run and a lot more in the medium run, breathes new life into both "third parties" and more rational forms and means of voting, makes U.S politics more diverse, competitive, and unpredictable, while basically being what Zizek would call a "vanishing mediator," a short-term catalyst for more fundamental changes (rather than, say, the opportunity to make Unity '08 into a real political movement);
  
  
• actively supporting Obama's candidacy in hopes that in doing so we can build a new party apparatus that in the long run can become a real political home for us.
  

Of course, different possibilities and risks would be raised for and by moderate Republicans, libertarians, and greens, to name a few, by an Obama/McCain Independent run. What do you all think?