John Judis has an analogy:
Anderson provided the margin of victory for Reagan in eleven states. Some of these were Southern states where Carter was strong, and Anderson got just a few percent, but others were the same kind of states where Democrats, with Independent votes, now win majorities. In these states Anderson got between 9 and 15 percent. They included Connecticut, Delaware, Massachusetts, New York, Oregon, Vermont, and Wisconsin. In addition, Anderson racked up large votes in Colorado, Arizona, Montana, New Hampshire, Hawaii, and Maine. Most of these are states that a Democrat needs to win to win the presidency. That's why the Democrats have more to fear from a Bloomberg candidacy than the Republicans.
I think this is true, but also potentially misleading. Jimmy Carter was an incumbent president widely judged to have failed in office (recall Ted Kennedy's very strong showing against him in the 1980 primaries). This created a large pool of people ideologically inclined to pick Carter over Reagan, but who also really didn't want to vote for Carter. Much the same could be said about people who defected from George H.W. Bush to Ross Perot; in both cases actual incumbent performance drove disaffection and a proclivity for third party voting.
It seems to me that this kind of dynamic is pretty uniquely associated with incumbency, and probably doesn't apply at all to the 2008 election and certainly doesn't apply to the Democrats in 2004.