E.J. Dionne, Jr. attempts to make sense of the Republican aspirants’ stances toward President Bush. He fails. According to him, Thompson and Giuliani have been least critical, while Romney (recently, at least), McCain, and Huckabee are trying to put the most distance between themselves and the President.
A better way of conceiving it is to think of four aspects of the Bush legacy: foreign policy, "compassionate conservatism," "the culture of life," and tax cuts (or, more broadly, fiscal and economic policy). Different parts of the Republican coalition embrace different elements of this legacy, and the candidates are appealing to these different parts. Thus, for example, far from being the most different from Bush, on "compassionate conservatism" and "the culture of life," Huckabee is most like Bush. And Giuliani has been least critical because he’s not about to say much about the one element of Bush’s legacy--"the culture of life"--where he’s most at odds.
It’s probably impossible to win either the nomination or the general election by frankly embracing all the facets of George W. Bush’s legacy. Romney tried that for a while (well, he didn’t say much about compassionate conservatism), but being all things to all people got him "silver medals," as he put it. And in a fractured field, everyone has to maximize his appeal to his niche. In this regard, it seems to me that McCain actually has the toughest row to hoe, because his crossover appeal to moderates and independents is at odds with his strong support for the U.S. presence in Iraq.
Nevertheless, however the nomination is won, and whoever wins it, I can’t imagine the nominee not trying to appeal to all the elements of the Bush coalition: national security conservatives, religious conservatives (whose agenda is indeed broadening, but not at the expense of bedrock socially conservative "values"), and fiscal conservatives.