The Republican civil war has so many factions, you need a scorecard to sort them out — tea partyers versus the establishment; conservatives versus moderates; gubernatorial wing versus congressional wing; religious rightists versus tolerants — and even the despairing Republican National Committee, in its newly released autopsy of the '12 campaign, says the party is "driving around in circles on an ideological cul de sac."

But surely the most significant fight is between the "stale and moss-covered" neoconservatives and the libertarian "wacko birds."

For more than half a century, Republicans have championed a muscular foreign policy, a robust military interventionism. This ethos has been embedded in the party brand since the Dwight Eisenhower era, it underpinned the party's anti-communist image, and, 10 years ago, it compelled the neoconservatives to blunder into Iraq. This ethos still has many adherents — most notably John McCain, and talking heads like Bill Kristol — but now there's a new player in town, and he's talking aim at the traditional Republican consensus.

I'm referring, of course, to Rand Paul.

As evidenced by his gig last Thursday at the Conservative Political Action Conference — where he strode on stage in jeans, to the accompaniment of music by Metallica — the freshman libertarian senator is clearly groovy. Groovy for a conservative confab, anyway. The young 'uns in the audience went wild for him. They waved "Stand With Rand" signs, a portent of what we might see in the '16 Republican primaries, and they loved it when he sneered at the likes of John McCain: "The GOP of old has grown stale and moss-covered — I don't think we need to name any names, do we?" That was payback for McCain's characterization of Paul and other intervention-averse conservatives as "wacko birds."

Name-calling aside, this is important stuff. Michael Gerson, who served as George W. Bush's chief speechwriter, was right the other day when he predicted that a Paul presidential candidacy would trigger "a lively debate on foreign policy fundamentals." If my history is correct, that hasn't happened since 1952, when Eisenhower faced off in the Republican intramurals against Robert A. Taft, a conservative leader and isolationist who opposed America's membership in NATO. (Indeed, Taft was the last of the prominent isolationist Republicans. Many had preceded him. North Dakota senator Gerald Nye fueled isolationism in the 1930s when he contended in a committee report that greedy banker and munitions dealers prompted our participation in World War I.)

Rand Paul is a throwback to that old GOP mindset. He broadly argues for a full American withdrawal from Afghanistan ("we're coming home") — and, more importantly, for a serious reduction in the American military presence abroad. He contends that isolationism should be a major feature of what he calls "the new GOP." There is indeed a constituency for his views, especially among younger conservatives. It's noteworthy that CPAC featured a panel entitled "Too Many American Wars? Should We Fight Anywhere and Can We Afford It?" Nobody at that event assailed President Bush as a disaster — Bush is still He Who Shall Not Be Named — but Paul's ascent is being fueled by libertarian conservatives seeking to distance themselves from Bush's tenure. Especially from an Iraq war that, at last glance, has run up a tab of two trillion bucks.

Hence the intramural Republican tensions. There are still plenty of neoconservatives, interventionists, and Iraq war apologists who think that Paul's isolationist ethos would be ruinous for the party.

Bill Kristol (natch) is one of them. Over the weekend, he called Paul a "McGovernite," which, in GOP parlance, is a slur one notch above pedophile. Kristol said that "the GOP of old, the GOP of Nixon, Reagan, and Bush, knew that while we may not be interested in war, our enemies remain interested in us." Meanwhile, A freshman congressman named Tom Cotton took a swipe at Paul during CPAC: "We're fighting ... a war against radical Islam and jihad...Do we have the will to win the war? Our enemies certainly have the will."

And Gerson, the ex-Bush speechwriter, says that the Bush and Obama adminstrations have rightly "found the threat of terrorism both real and unappeasable. In this period, the American government, with congressional authorization, has destroyed terrorist training camps; undermined terrorist communications, fundraising and planning; targeted terrorist leaders; and disrupted at least 40 plots aimed at U.S. targets. Far from perpetrating imaginary terrors on Americans, the government has protected them from real ones. Which is the reason that ­Republicans, in the end, cannot ­Stand With Rand."

Even if Paul does not run for president (he's reportedly gearing up), his vocal isolationism might serve a useful purpose. If he can prompt an honest intramural debate about the limits of interventionism, if he can force Republicans to finally own up to the bloody and costly mess they made in Iraq, perhaps the party can recalibrate for the future — interventionism, minus the foolish adventurism. But if Paul succeeds all to well in his argument for a reduced American commitment abroad, the GOP risks ceding the foreign policy center to the Democrats — just as it did in 2012, thanks to the feckless Mitt Romney. Rest assured, Democrats everywhere are hoping that the other party Stands With Rand.

And bet on this, going forward: Worried interventionist Republicans will target Paul in terms far stronger than "wacko bird."

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