Sen. Rand PaulU.S. SenateLast Thursday, about 30 hours before President Barack Obama abruptly changed his mind about seeking congressional approval to bomb Syria, I asked Sen. Rand Paul (R-Kentucky) about how the whole Wacko Birds vs. Angry Birds conflict within the GOP was playing out in Washington.

"We're losing, on a good day, 70/30 among the Republicans," Paul said. "But we win every day among the grassroots, probably 80/20, 90/10."

This coming week, which promises to be the most important and bitterly contested conflict of governing visions on Capitol Hill since the September 2008 votes on the Troubled Assets Relief Program, will put Paul's math—and lobbying abilities—to the test.

Like TARP, this battle features a second-term president rallying a do-something coalition of big-government conservatives, liberal interventionists, and D.C. establishmentarians against a less telegenic mix of Republican hotheads and progressive Democrats. As in 2008, Sen. John McCain (R-Arizona) is playing a key role in supporting the White House and warning about the "catastrophic" consequences of inaction.

Only this time, there is a new political bloc called the Liberty Movement, comprised of GOP politicians such as Rand Paul and Rep. Justin Amash (R-Michigan) who came to office in opposition to both TARP and the imperial presidency, now leading the charge against Washington interventionism as usual.

As The New York Times put it Monday, "Republican divisions on national security have flared over the use of drones, aid to Egypt, and the surveillance practices of the National Security Agency, and the tensions have played out publicly in battles between Senator John McCain of Arizona, the former Navy pilot and Vietnam prisoner of war, and Senator Rand Paul of Kentucky, the libertarian-leaning freshman."

McCain famously coined the term "Wacko Birds" in March to describe Paul, Amash, and Sen. Ted Cruz (R-Texas) after Paul's historic 13-hour filibuster in protest of the Obama administration's drone policy. Paul since then has continuously referred to the McCainite wing of the GOP as "stale and moss-covered," suggesting that National Greatness Conservatism is yesterday's news in an increasingly libertarian GOP.

"Between Ted and [Utah Sen.] Mike [Lee] and I, that nucleus, I think, is becoming less the Wacko Bird conference, and more the future of the party," Paul told me. Or as Amash Tweeted yesterday, "GWB-era foreign policy is nearly extinct among GOP grassroots. Some Rs in DC either didn't get the memo or haven't been home in a while."

Since the drone filibuster, Paul has come under repeated attacks from the likes of putative 2016 GOP presidential candidates Chris Christie and Rep. Peter King (R-New York), the latter of whom has cited Paul's views on foreign policy and civil liberties as motivation for his potential candidacy. On Fox News Sunday this week, King, normally one of Obama's fiercest critics, took the extraordinary step of both supporting the Democratic president and lashing out at the Republican opposition: "Hopefully, the president can make his case, that he will be able to get a majority of the House of Representatives," King said. "Right now, it would be difficult. Also, we have an increasing isolationist wing in the party which I think is damaging to the party and to the nation."

GOP interventionists have been open to preferring Democrats to Rand Paul. McCain in a July New Republic interview said that a Paul vs. Hillary Clinton presidential race in 2016 would be a "tough choice" for him. Last week, Washington Post "Right Turn" blogger Jennifer Rubin concluded a piece titled "Why Rand Paul should never, ever be commander in chief" with this comment: "A Capitol Hill Republican mused that perhaps Rand Paul is concerned that Sen. Ted Cruz (R-Tex.) is 'stealing his thunder' on foreign policy isolationism. Maybe. In any event, their utterances demonstrate clearly why America would be at risk (more so than even under Obama) if either got into the Oval Office (except on a tour)."

This is all just prelude to the rhetorical pugilism likely to come out over the next seven days. But refreshingly for Washington, the dispute is anchored firmly in mutually opposing philosophy. Rand Paul and Justin Amash believe in a "constitutionally conservative" approach toward war, one that requires—instead of occasionally asking for—congressional approval before launching U.S. military action. They are far more skeptical about intervention, military or otherwise, than any other bloc in Congress. They worry constantly that the War on Terror has dangerously eroded constitutional liberties. And they think the last century of American history has featured a dangerous over-concentration of power in the executive branch. In every one of these beliefs, they stand in sharp contrast not just to John McCain and the neoconservatives, but to the dominant tendencies in Republican foreign policy since at least September 11, 2001, and arguably for decades before that.

"The president as commander-in-chief has absolute constitutional and statutory power to take military action," King insisted on Sunday. (Note the word "absolute.") McCain warned that with a no vote, "the credibility of this country with friends and adversaries alike would be shredded, and there would be not only implications for this president, but for future presidencies as well." And at The Weekly Standard, Editor William Kristol wrote a blog post recommending a James Ceaser essay that begins with this remarkable passage:

Republicans should support some version of the authorization of force resolution. They should do so even if they think that the President’s policy will prove ineffective, do no good, waste money, or entail unforeseen risks; they should do so even if they think he has gotten the nation into this situation by blunders, fecklessness, arrogance, or naiveté; and they should so even if, and especially, if they have no confidence in his judgment. The simple fact is that the nation and our allies will be at further risk if the world sees a presidency that is weakened and that has no credibility to act.

This important philosophical divide, which Reason has been tracking for a long time now, has been tilting steadily these past six months in the Liberty Movement's direction. First Rand Paul's filibuster made national headlines and hinted at a new bipartisan coalition of civil libertarians. Then a series of leaks originating from NSA whistleblower Edward Snowden repeatedly put the lie to 12 years' worth of White House claims about the surveillance state. As a result, for the first time since pollsters have been asking the question, more Americans fear government’s encroachment on their civil liberties than they do a terrorist attack. Meanwhile Obama's poll numbers among tech-savvy young voters has taken a double-digit tumble.

The biggest legislative moment in this shift—at least prior to this week's Syria debate—occurred at the end of July, when Justin Amash came within just 12 votes of passing an amendment to substantially roll back the powers of the NSA. Most worryingly for interventionists of both parties, support for the Amash amendment was bipartisan: 94 Republicans and 111 Democrats voted aye. When you consider that more than 70 of those yes votes co-signed a letter to Obama last week demanding a congressional vote on Syria, the president's sudden flip-flop toward the Constitution looks a little less surprising. When a budding bipartisan coalition is aligned with a 79 percent majority of public opinion, that's an opportune time to remember the principles that vaulted you to office (before they were dropped like a hot rock).

So in many senses, Liberty Movement politicians, and the voters supporting their efforts, have helped create the reality that Obama is grudgingly responding to. That is a noteworthy feat on its own. But the real fight, cutting straight to the heart of life-and-death policymaking, is now upon them. And us.

As far as the Wacko Bird contingent has moved the needle on American politics the past three years, and particularly the past six months, they have yet to win a vote that truly matters. Usually, as Paul said, they are on the losing side of 70-30 battles. Already, House Speaker John Boehner (R-Ohio) and Majority Leader Eric Cantor (R-Virginia) have announced their support for the Syria authorization, leaving House Republican opposition up to a ragbag group of upstarts led by Amash.

Amash, frequently touted as the House's successor to Ron Paul, has been poking his thumb in Boehner's eye ever since last November's election, for which he has been repeatedly slapped down. For now he is ringleading mostly from afar, holding a series of town halls in Michigan on Syria and lobbing the occasional social-media taunt in Boehner's general direction about the persistent unpopularity of a Syrian war.

It is in that latter phenomenon, and not in any kind demonstrated skill at arm-twisting, that Rand Paul and Justin Amash will ultimately win or lose this momentous struggle. Leadership is bound to lead (or do the Washington facsimile thereof, which is to bury the partisan hatchet and support the president's war), but backbenchers have to face disgruntled electorates not just in November 2014 but in possible primary fights before. Will the same limited-government, primary-exploiting activists who brought the Liberty Movement into existence be willing to punish Republican congressmen for favoring unpopular wars? So far there's no hint of any such thing.

As in the TARP vote, that leaves a disorganized group of people's representatives in the uncomfortable position of opposing the establishment while better representing Americans' views. Surely, if they vote no, they will be called "nihilists" and "appeasers" and much worse. Sen. Paul, for one, is confident that public support will protect the anti-interventionists' political fortunes.

"We have 50,000 soldiers at Fort Campbell," he says. "If you were allowed to do a poll of them, and ask 'How many of you guys are ready for your next tour in Syria?' It would be zero."