The latest Fox News/Opinion Dynamics Pol; (The margin of error for the GOP breakout of the poll is +/- 6%.)) has a three-way tie at the top: Giulani (20%), Huckabee (19%) and McCain (19%). The PDF of the full poll can be found here.
Here’s the GOP section specifically:

The most interesting, and telling, bit of information is that Giuliani lost 13 points from November to December. Likewise, Huckabee’s explosion is clear and dramatic. McCain, apart from an August slump, has been fairly steady. Romney, too, has been steady, which is problematic for him, as the time is now to make a move. Thompson’s fade continues as well.
Two other points: Ron Paul supporters, please note: Dr. Paul is included in the poll, and is in a statistical tie with Duncan Hunter and the now-out-of-the-race Tom Tandredo. And, I must confess, I don’t see a Huckabee-like take off any time soon (indeed, I expect the energy of Huckabee’s ascent to exhaust itself soon).









