Charles Ommanney / Reportage by Getty Images for TIME Republican Dean Young campaigns at the Williams Station Day County Fair in Atmore, Ala., Oct. 26, 2013.

The fight for the future of the Republican Party has begun.

Deep in the heart of the South, a Republican runoff for the vacant seat in Alabama’s First Congressional District has emerged as a proxy war between the GOP’s business wing and a Tea Party movement that saw the government shutdown as just the first step in the right direction. The safe-seat skirmish between Bradley Byrne, a conservative former state legislator, and Dean Young, an arch-conservative Tea Party upstart, has taken on national import, as I write in this week’s issue of TIME.

On Thursday, less than a week before the Nov. 5 Republican runoff, voters across Alabama’s First Congressional District will open their mailboxes to find a double-sided flyer, a copy of which TIME obtained exclusively. At first glance, the mailer may seem like a standard plug. But it says plenty about both the dynamics of the jigsaw-puzzle district, as well as the national battle that is erupting for the soul of the party.

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First, note the source. The U.S. Chamber of Commerce is the titan of Washington business groups, which were deeply dismayed by the government shutdown. The Chamber spent more than $35 million in 2012, and intends to mount a “vigorous” effort across Republican primaries next year in support of business-friendly allies, the group’s president, Tom Donohue, told reporters in Washington recently. On Oct. 29, it became the second major national organization to endorse Byrne, following the NRA. A series of local organizations and a spate of political officials have also thrown in their lot with the former lawmaker, who came up short in his 2010 bid to win the governorship.

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The mailer declares Byrne a “strong conservative.” One reason the Chamber applies the label is that opponents say otherwise. A former Democrat—a not-uncommon skeleton for conservatives in the old South—Byrne once donated to Bill Clinton (“the worst mistake of my life,” he says) and as a legislator took a vote that raised taxes a decade ago. But “by every reasonable standard,” says Quin Hillyer, a conservative journalist who finished fourth in the primary before endorsing Byrne, “he is a solid Reagan conservative.” The differences between two candidates’ policy positions on Obamacare, spending, taxes and social issues are small. As in Washington, the difference between the so-called GOP establishment and the conservative counterpart is largely a question of tactics and tone.

Byrne’s conservatism—and for that matter, the candidate himself—is a secondary consideration on the flyer. His smiling face is smaller than House Democratic leader Nancy Pelosi’s. This is no accident. The subtext suggests that a vote for Young could jeopardize the House Republican majority, which was shaken slightly by the shutdown fiasco. And there is some merit to the theory.

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Young likes to boast that he would be “one of the most conservative members of Congress,” and there is no reason to doubt the claim. “I’m a Ted Cruz guy,” he says, adding of Byrne: “He’s a Mitch McConnell guy.” Young’s reverence for the Texas provocateur, and his record of overheated rhetoric, have stirred fears even among movement conservative stalwarts. While the GOP establishment is all-in for Byrne, across the great Republican divide, Tea Party allies have been conspicuously absent.

The Club for Growth took a look at the race and evidently decided not to jump in. Ditto the Madison Project. According to multiple sources inside the Tea Party movement and in Alabama, grassroots groups were put off by Young’s ramshackle campaign and lack of polish as a candidate. (One notable exception: Our Voice PAC, an outfit led by former Nevada Senate candidate Sharron Angle—the kind of insurgent the Club and others are desperate to forget—plunged in with TV ads for Young.) At a debate in Mobile, I watched Young flub a simple question about the recent Supreme Court decision on the Voting Rights Act, a critical issue in the South.

It is also possible that national conservative groups were put off by Young’s long history of comments opposing gay marriage. During the nine-way primary, he tried to coax fellow Republicans into signing a pledge to oppose the practice, which he told TIME was “ripping apart the moral fabric of America.” This stance is common in lower Alabama; indeed, it may have helped propel Young into the runoff. And it has long been a central tenet of the GOP, codified into the party platform as late as last summer. But now that more than half of Americans support marriage equality, a candidate with a deep-seated and vociferous opposition to gay marriage may not be what national groups have in mind.

There is no question Young feels jilted. “The establishment organizations are getting behind Bradley Byrne,” he says, whereas the national Tea Party groups “are not showing up.” He believes their absence is telling. “This is a bellwether election,” he says. “If I win, the Tea Party is alive and well.”

And if he loses? “If we can’t win in South Alabama,” Young drawls, “the Tea Party needs to hang it up.”

Correction appended, Oct. 31: This post originally misidentified Bradley Byrne as the candidate who botched the debate question about the Voting Rights Act.

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Click here to join TIME for as little as $2.99 to read Alex Altman’s story on the emerging 2014 battle between establishment and Tea Party Republicans.