Patrick Ruffini is running a straw poll to see who is the front-runner for the 2008 Presidential nomination for the Republican party. The results at the moment are interesting.
Among serious contenders, Guiliani is leading. Among a larger list of (more unlikely) candidates, Rice and Jeb Bush do very well.
Here's the problem as I see it. Guiliani probably has the best chance to appeal to the widest group of people. But he is "liberal" on several key issues and the religious right is unlikely to support him, choosing instead to do follow their standard "stay home" voting strategy. The loss of that segment of Republican voters, the name recognition of someone like Clinton, and the unleashed anger of the loony left after eight years of W spells disaster in the 2008 election for the Republicans.
A lot could change between now and then of course. Future events could make someone new a clear choice for the nomination. Or the fringe right of the party could realize that the best chance of keeping control of the presidency is to support someone with mass appeal, such as Guiliani. But if (as I suspect) the current leaders of the Republican party become arrogant from their sustained success over the last few decades and continue to push more to the right, they will be very vulnerable to a legitimate candidate from the Democratic party. Time will tell.