Writing on TNR's Open University blog, University of Texas professor Sanford Levinson proposes a Rudy Giuliani third-party run (rather than the more conventional call for an independent McCain candidacy):
The two-party system remains vulnerable to a challenge, and Rudy, for better and, definitely, for worse, is precisely the kind of person who could explode a lot of verities. He would offer Republicans who are sick and tired of the stranglehold over their party of the religious right a "safe harbor," without having to vote for, say, Hillary Clinton. Ditto those (unlike myself) who admire Lieberman and feel his pain at the ostensible leftward drift of the Democratic Party. One can easily imagine such a ticket winning enough electoral votes to throw the election into the House (the long-awaited train wreck), and there are even scenarios, albeit longshot that could have them winning enough of the large states plus some others to take the whole thing.
But what states would Giuliani and Lieberman win? Voters aren't idiots. If they know that the major party tickets are going to be the top two contenders, the logic of "wasted votes" (ie strategic voting) will cause many Giuliani supporters to shift to their second choice, which has a better chance of winning. You can partially overcome that dynamic as a one-issue protest candidate (a la George Wallace) but Giuliani has no such issue to run on. And a Rudy third-party candidacy would undermine the lucrative consulting gigs and speaking engagements that he is currently getting from the GOP establishment.