Mitt Romney's limited prospects

Recent polls indicate that Mitt Romney's support is rising among the Republican pack. The hypothesis that Giuliani’s electability and his muscular rhetoric in the war on terror would win over conservatives may be incorrect, perhaps his divergence from the Republican base on social issues will sink him, whereas McCain despite his conservative record on policy suffers from his estrangement from the 'conservative movement'. Doubts about the sincerity of Romney's conversion to conservative position inspire the search for alternative candidates, hence the curious spectre of significant support for Fred Thompson, even although he is not an official candidate. Maybe Romney will win the nomination. However then his problem will be not just his religion but the problem that that he will be an identikit Republican candidate in a Democratic year, to win the Republicans need a candidate with a broad appeal. It is a concern that both Giuliani and McCain have done well in match-ups against potential Democratic candidates but Romney does poorly. This conclusion is supported by a recent Pew analysis that suggests that mcCain and Guilani's strong performance in match-ups suggests that:
crucial personal dimension in a period of national discontent, is whether the candidate is seen as an agent of change. And at this early stage in the game, the Republican front runners might just fill that bill. A recent Pew survey found that most voters make a big distinction between both John McCain and Rudy Giuliani and President Bush. Both candidates are seen as less conservative than Bush, and much closer to the average voter's own political beliefs. ..Another piece of evidence for the potential appeal of the Republican frontrunners is the support they draw from political independents--the group that was so eager for political change in 2006, and played a decisive role in the Republican congressional defeat. A recent Pew poll found that as presidential candidates both Giuliani and McCain were about as appealing to independents as were Clinton and Obama, even though a plurality of independents say they lean Democratic these days...Of course, the very appeal of Giuliani and McCain as more centrist and politically distant from Bush threatens their viability in the GOP state primaries races where independents are often barred from voting and voters with strongly held conservative beliefs are most likely to turnout. Indeed, Republicans in Pew's survey placed themselves very close to President Bush on the liberal-conservative continuum and quite a bit to the right of where they placed the candidates they now say they are most likely to support for the Republican nomination...The message of the horse race polls for the Republican Party may be that while McCain, and Giuliani might be perceived as insufficiently conservative for a majority of GOP voters, ultimately only they, or someone else with centrist appeal, may be able to hold off the broad advantage the Democrats have going into this election. For the Democrats, the message may be that, while there is broad discontent with Bush, which has hurt his party, their own potential nominees are not so strong that they can rule out being beaten by a Republican who is seen as an agent of change.
Could Peter Costello appear as an agent of change?