Hugh Hewitt explains why with Gingrich out, the Republican nomination will come down to a two-way race between McCain and Romney:

Speculation: My guess is that most of the Gingricj [sic] voters go to Romney, as they must represent the most conservative voters and the ones which want the idea factory open.  Some will go to the other two of course, but with the Gingrich decoy out of the race, it is almost certainly a tie between Romney and McCain, with Giuliani backing up.  The debates are going to mean a great deal, and a great deal earlier than ever before.

While Romney-booster Hewitt bases his analysis on guessing and speculation, I'd prefer to look at hard data, which, unsurprisingly, directly contradicts Hewitt's assertion. Today, Gallup released an aggregate of its February and March polls, splitting the Republican electorate between conservatives and moderate/liberals.

By isolating the numbers among self-identified conservative Republicans, both with and without Gingrich, I compiled this table:

                        W/ Gingrich        W/out Gingrich    Change

Rudy Giuliani           38%                   43%               +5%

John McCain           20                      21                   +1

Newt Gingrich         14                       —                    —

Mitt Romney             8                       11                  +3

As you can see, with Gingrich out of the race, Giuliani actually extends his lead over Romney and McCain among conservatives.

Why might this be the case? One explanation could be that Gingrich's appeal is comprised of conservative voters who have fond memories of him from the 1994 Republican Revolution and conservative voters who like his stance on the War on Terrorism. So, it makes sense that the constituents who place more emphasis on terrorism would tend to gravitate toward Giuliani. Furthermore, Hewitt argues that Gingrich supporters "want the idea factory open" and takes it for granted that those voters will support Romney. However, one thing that a lot of pundits don't realize about Giuliani is that he's quite the wonk himself, and after all, Gingrich did choose to co-author a recent Wall Street Journal op-ed with Giuliani.

With Romney stuck in the single-digits, and Gingrich pretty consistency polling in the low to mid teens, Romney's supporters are fantasizing about the prospect of Newtniks  defecting to the former Massachusetts governor, because he needs to consolidate conservative support to compete with Giuliani and McCain. As long as Giuliani takes a bite out of the Gingrich vote (and this poll indicates it could be a significant bite), it'll make it virtually impossible for Romney to win, given Giuliani's support among moderates.

Some may counter that it's still early, polls at this stage are meaningless, will change once conservatives learn more about the candidates and warm up to Romney's "evolution" while cooling on Rudy because of his liberal social views. But at the moment, that's all guesswork.