You can read John McCain's Foreign Affairs essay here. Notice the title - "An Enduring Peace Built on Freedom" - and this paragraph:
We should go further by linking democratic nations in one common organization: a worldwide League of Democracies. This would be unlike Woodrow Wilson's doomed plan for the universal-membership League of Nations. Instead, it would be similar to what Theodore Roosevelt envisioned: like-minded nations working together for peace and liberty. The organization could act when the UN fails - to relieve human suffering in places such as Darfur, combat HIV/AIDS in sub-Saharan Africa, fashion better policies to confront environmental crises, provide unimpeded market access to those who endorse economic and political freedom, and take other measures unattainable by existing regional or universal-membership systems. This League of Democracies would not supplant the UN or other international organizations but complement them by harnessing the political and moral advantages offered by united democratic action. By taking steps such as bringing concerted pressure to bear on tyrants in Burma (renamed Myanmar by its military government in 1989) or Zimbabwe, uniting to impose sanctions on Iran, and providing support to struggling democracies in Serbia and Ukraine, the League of Democracies would serve as a unique handmaiden of freedom. If I am elected president, during my first year in office I will call a summit of the world's democracies to seek the views of my counterparts and explore the steps necessary to realize this vision - just as America led in creating NATO six decades ago.
Contrast McCain's "League of Democracies" with Giuliani's idea to expand NATO, regardless of the new member states' democratic status:
America has a clear interest in helping to establish good governance throughout the world. Democracy is a noble ideal, and promoting it abroad is the right long-term goal of U.S. policy. But democracy cannot be achieved rapidly or sustained unless it is built on sound legal, institutional, and cultural foundations. It can only work if people have a reasonable degree of safety and security. Elections are necessary but not sufficient to establish genuine democracy. Aspiring dictators sometimes win elections, and elected leaders sometimes govern badly and threaten their neighbors. History demonstrates that democracy usually follows good governance, not the reverse. U.S. assistance can do much to set nations on the road to democracy, but we must be realistic about how much we can accomplish alone and how long it will take to achieve lasting progress.
Among the top four candidates for the GOP nomination who have contributed these essays to Foreign Affairs (Thompson hasn't yet), only McCain embraces the idea that democracy promotion should be a top concern of U.S. foreign policy. Indeed, when I searched for the word "democracy" in Mitt Romney's Foreign Affairs essay, it came up exactly once. The Iraq war and Hamas's victory in the 2006 Palestinian elections have left a lot of Republicans feeling weary towards the Bush Doctrine. To varying degrees, they downplay America's role in fostering democracy abroad in favor of typically "realist" positions. But does that sort of foreign policy speak to American ideals and resonate with GOP voters? I'm skeptical. And what would happen to those candidates who have distanced themselves from the Bush Doctrine if the situation abroad in 2008 is different from what it was in 2006?