Senators Mitch McConnell (left) and Rand Paul

FORTUNE — It’s easy to understand why Rand Paul, Kentucky’s junior senator, is his state’s breakout political star. On Wednesday afternoon, I was on hand to see the 51-year-old libertarian Republican at Berkeley — hardly a right-wing redoubt — bring a crowd of college students to their feet not once but twice with a speech assailing the federal surveillance regime. It was a stunt that’s becoming a Paul trademark: Marching into the heart of what should be unfriendly territory to upend expectations, including by calling out his own party. As he explained it to the crowd on Wednesday (following prepared remarks that quoted both a 16th century Christian martyr and Pink Floyd’s Roger Waters), his appearance extended in part from his belief that the GOP has to “evolve, adapt, or die.” He said Republicans need to borrow a strategy from Domino’s Pizza and acknowledge they’re pushing an inferior product. “Maybe then people would say, ‘I always hated those Republicans, and their crust sucks, but maybe there’s some new Republicans.’”

And it likewise doesn’t take a giant imaginative leap to think the figure Paul has in mind when he’s agitating against an outmoded, entrenched, disconnected GOP is his own senior senator, Senate Minority Leader Mitch McConnell. Paul won his seat in 2010 as a political neophyte who rampaged over a candidate with McConnell’s blessing and the backing of the Kentucky Republican establishment that McConnell himself had built. And while the two since have formed a solid working relationship, they represent both in style and substance the poles of a party in the throes of a struggle to redefine itself. On sizzle, Paul wins in a walk. But anybody interested in the outcome of the Republican identity crisis needs to get to know McConnell. The 72-year-old is a canny survivor who’s half-century in public life traces the arc of the modern GOP — from a moderate Senate aide in the 1960s, to Reagan foot soldier as a young Senator in the 1980s, to chief Obama antagonist today. Despite the accommodations he’s made in recent years for an insurgent Tea Party movement, McConnell remains the Republican establishment’s Last Great Hope.

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Yet now, with Republican control of the Senate in reach — and with it, his career ambition of becoming Majority Leader — McConnell is also as close as he’s been in three decades to being sent packing from the chamber. He’s got to sink a primary challenge from the right by multimillionaire investor Matt Bevin before turning his full attention to what’s expected to be one of the hardest fought, most expensive, and nastiest general election races in the country against Democrat Alison Lundergan Grimes, the 35-year-old Secretary of State. In the process, McConnell has declared war on the constellation of outside groups promoting Tea Party challengers to incumbents across the country. If he “crushes” them, as he’s pledged, and emerges victorious in November, he could well captain a Republican majority more recognizable to the Chamber of Commerce types who’ve lately felt like strangers in their own home. A loss, on the other hand, would extend the muddle of GOP infighting, with potentially disastrous consequences for the party heading into the 2016 presidential election.

If anyone can pull it off, it’s McConnell. As he once told an associate, “It’s amazing what you can do when your back’s against the wall.” After all, this is the guy who in November 2008 rallied Republicans still sorting through the rubble of an electoral thrashing to begin plotting their response to Obama’s health care reform push — two months before the new president was even inaugurated. It worked, of course, and now Democrats are on the defensive over the landmark law heading into the midterms. Likewise, McConnell positioned himself as Wall Street’s indispensable man during the debate over bringing the industry to heel. He met with billionaire New York City Mayor Mike Bloomberg to hear his complaints about being orphaned by his home-state Democratic Senators who supported reform. And at a private New York fundraiser, he counseled an insurance industry executive about how to achieve a key fix in the bill from a Democratic Senator. McConnell’s only stipulation: “Don’t let [him] know I told you to go to him.” All the while, the Kentuckian has proven himself a peerless master at separating lobbyists from their bankrolls — an unglamorous task that nevertheless can tip the balance during the cash-burning campaign slog. At one series of dinners at an Italian restaurant in downtown Washington, McConnell, in Godfather-mode, quietly cajoled K Street heavy-hitters who thought they were only there for spaghetti and meatballs into forking over five-figure checks — events that organizers called “the sandbag dinners.”

For more on McConnell, read Why Mitch McConnell really matters and The secret to Mitch McConnell’s millions from the April 7th issue.