WASHINGTON D.C. (Fortune) -- Rahm Emanuel, the Chicago Congressman who will be President-elect Barack Obama's White House chief of staff, is "dangerous, absolutely relentless when he's got a political kill in sight," according to an admiring Republican colleague, Oklahoma Congressman Tom Cole. Emanuel is also, President Clinton once told Fortune, "one of the top political minds in Washington," a former ballet dancer who "favors the counterattack over the attack."

From his years at Clinton's side, Emanuel knows his way around the West Wing - and was a critical figure behind the passage of NAFTA. From his years in the U.S. House of Representatives, he knows his way around Capitol Hill - and was the chief architect of the Democrats' 2006 recapture of Congress.

Find out how Emanuel's life was shaped by a near-death experience in high school and lifelong competition with (and enduring love for) two equally successful brothers in this 2006 profile by Fortune's Nina Easton.

On a wretchedly hot August day outside the Caterpillar tractor plant in Montgomery, Ill., President Bush and the state's congressional delegation gather for the signing of the massive transportation bill. This is 2005, the calm before the Katrina storm, and a rigorous mountain-biking schedule has the President in top shape.

In off-camera chitchat with the shirt-sleeved lawmakers, Bush takes note of Democratic Congressman Rahm Emanuel's deep tan, prompting the 46-year-old Emanuel to boast about the miles of swimming and biking in his triathlon training schedule.

(This story is an excerpt from the story that ran in the October 2, 2006 issue of Fortune. Read the full story.)

Testosterone oozes into the humid air space between the two men. Bush invites Emanuel down to Texas to do some real biking. "So I said, 'I'll make you a deal, Mr. President. I'll bike if you swim.' Now he didn't exactly say swimming was a wussy sport, but you could tell.... So I said, 'Mr. President, Laura can put your water wings next to the lake. You can have your water wings.' "

At that point you might think this graduate of the Evanston School of Ballet would leave well enough alone. But Emanuel is hard-wired to go for the jugular: Politics Chicago-style are part of his DNA. So he sharpens his drill bit on the leader of the free world. "I said to him, 'You're not one of those tribathletes, are you, Mr. President? You know - steam, sauna, shower?'

"And Bush goes, 'That's g-o-o-d.'"

Banter with a U.S. President is nothing new to Emanuel; he was at Bill Clinton's side as a political advisor inside the White House for six years and still talks strategy with him at least once a month. Now chair of the Democratic Congressional Campaign Committee - the operations center for House candidates - Emanuel is applying rugged business discipline to the Democratic Party's historic effort to wrest control of the U.S. House from the Republicans. Last year he recruited dozens of candidates to challenge GOP incumbents. This year he is holding feet to the fire to raise record amounts for the Democrats' effort.

Along the way Emanuel has widened his core of admirers - and made powerful enemies. Nervous about being swamped by Republican money this fall, he spent the summer locked in a bitter dispute with Democratic National Committee chair Howard Dean over the allocation of election resources. In private Emanuel told off Dean. In public he's aimed similar messages at liberal financiers like George Soros for being stingy and at the leftist activists in MoveOn.org for being ineffective.

All this matters, of course, only if the Democrats lose. "Holy Christ, his butt is on the line," says Democratic strategist Paul Begala, who describes Emanuel's aggressive style as a "cross between a hemorrhoid and a toothache."

Emanuel has rejuvenated the hopes of House Democrats in no small part by applying the money-raising acumen he used when Clinton's 1992 presidential campaign nearly sank under the weight of Gennifer Flowers' accusations. With a penchant for networking and making deals - honed during a stint at Wasserstein Perella (which netted him more than $18 million in just over two years) - Emanuel has put the party's House campaign coffers on a par with the Republicans' for the first time in years.

As the fall campaign clicks into high gear, Democratic strategists say they have at least a fifty-fifty chance at taking back the House. Emanuel's fears about being slaughtered by RNC money are legitimate. A deal he brokered with Dean in early September committed the DNC to spend $2.4 million on 40 competitive House races, while the RNC will be drawing on a war chest that will probably clock in at 25 times that size.

Emanuel's in-your-face money demands make him stand out in a party that has sometimes been a little prissy about big-donor fundraising. He's expanded the DCCC's donor base, appealing both to like-minded young financiers and big-business donors with GOP ties who are hedging their bets this fall.

When it comes to slicing and dicing his Republican foes, Emanuel applies a Chicago pol's sensibility that recalls that famous "Untouchables" line: "He pulls a knife, you pull a gun; he sends one of yours to the hospital, you send one of his to the morgue."

Mostly out of power for the past six years, the Democrats could use Emanuel's comeback instincts. So the match is a timely one.

This story is an excerpt from the story that ran in the October 2, 2006 issue of Fortune. Read the full story.

Research associate Joan L. Levinstein contributed to this article.