It seems some Democrats cannot believe Giuliani is the front-runner.
They need to believe it. In some areas, Giuliani is a welcome change from Geroge W. Bush. He is the type to engage in bare-knuckle fighting when necessary - just look at his comments about Hillary Clinton and Barack Obama. George W. Bush, for all the good stuff he has done, just never had his heart into a brawl - and the same could be said for Mitt Romney to a degree.
So, a lot of Republicans are putting disagreements with him on some issues aside. Primarily because he is their strongest candidate, but also because he does agree with them on the big issues (war on terror and economics). The social conservatives' strategic incompetence over the last two-plus years has also benefitted Giuliani (more on that later).
In essence, this has been Giuliani's to lose for a long time.