Check out the numbers at Michael Barone's Blog:
***...Conclusions: This confirms Giuliani's significant rise from the 27-to-30 percent level of November to January to the 35-to-37 percent level of February to March. Also, McCain's fall from 25.5 to 27 percent in November to December to 20 to 23 percent in January to March. Also, note that the percentages for others and undecideds have been declining, roughly in tandem with the move to Giuliani; Romney's numbers have not had a significant change, and neither, it seems, have those of Gingrich, who in any case is not a declared candidate. Yes, there's a market out there for another alternative. But it's not a market that has been expanding....***
http://www.usnews.com/usnews/opinion/baroneblog/archives/070321/a_look_at_the_p.htm
The big money players on the Republican side are betting on Giuliani because he is looking more like a winner as McCain implodes, Romney falters, and the conservatives are divided among several second tier candidates.