Jonah Goldberg writes that 9/11 has caused a remarkable shift in concerns amongst the Republicans' conservative base, which will be played out in the race for the nomination between former Massachusetts Gov. Mitt Romney and former New York City mayor Rudy Giuliani:

Romney was a dedicated pro-choice politician for most of his career. When he ran against Ted Kennedy for the Senate, he was as pro-choice as you can get.

Now, at least partly to win over social conservatives, Romney is unapologetically pro-life, saying that he realized the folly of his ways when dealing with embryonic stem cell controversies as governor. I have some quibbles with his conversion story, but that's a subject for another column.

Then there's Rudy. He's going a different way. While tacking and trimming somewhat, he's basically staying pro-choice. Whatever his true convictions, the simple fact is that he has little choice. Unlike Romney, who had the stem cell controversy as an impetus for his conversion, Giuliani - who once almost went into the priesthood - now has no plausible excuse to switch positions even if he wanted one. You need some story, some event, to believably pull off a switcheroo of that proportion, and running for president isn't one of them. So, while he's saying the right - and Right - things about judges and judicial restraint, he's not backing off.

It seems indisputable that prior to 9/11, Romney's strategy would win and Giuliani might not even bother trying.

Of course, Giuliani's national profile expanded enormously because of 9/11. And while the press harps on that point, the more interesting part of the story lies elsewhere. The war on terror hasn't just changed Giuliani's profile as a crisis-leader, it's changed the attitudes of many Americans, particularly conservatives, about the central crisis facing the country. It's not that pro-lifers are less pro-life or that social conservatives are suddenly OK with homosexuality, gun control and other issues where Giuliani's dissent from mainstream conservative opinion would normally disqualify him. It's that they really, really believe the war on terror is for real. At conservative conferences, on blogs and on talk radio, pro-life issues have faded in their passion and intensity compared with the war on terror. Taken together, terrorism, Iraq and Islam have become the No. 1 social issue for conservative base of the party.

Note: I didn't say it's become the No. 1 foreign-policy or national-security issue for social conservatives. It's become the No. 1 social issue, at least for many of them.

Unambiguous polling data is hard to come by on this point, but the anecdotal data is enormous.

That's not to say either Romney or Giuliani will win, but they're the ones to watch because they get to design their first impressions in a way other top-tier candidates like John McCain and Newt Gingrich can't. Romney and Giuliani, both immensely attractive, savvy and well-funded politicians, are in effect the canaries in the coal mine of conservative politics. If either emerges from the dark tunnels of primary season alive, it will tell us a great deal about the future of the GOP and American politics.