Giuliani has the least chance of any Republican of winning the general election. All one has to do is look how far behind he is in fundraising compared to the Democrats to figure that out.
A candidate that results in millions of his own party staying home or not being enthused about his candidacy has very little chance of winning a general election.
A June 5th Harris poll:
One of the lasting criticisms of the Mayor has been whether his liberal past would hurt him with Conservatives. This may very well be the case, as just half of Conservatives would vote for him while one-quarter (26%) would not and 21 percent are not sure.
It all depends on how many independents Giuliani can pick up. The 2008 general election will most likely be determined by moderate independents. Giuliani is the strongest Republican candidate among that group. If Giuliani couldn't offset the likely conservative defections with these moderates, you would be correct that he would have little chance.