Bill Ackman’s friends describe him in two ways. They offer the euphemism that the prominent hedge fund manager “does not suffer from low self-esteem.” Then they observe that he is optimistic—almost clinically so. A pop psychologist might diagnose Ackman with hypomania, a condition notable for persistently elevated moods but without the self-destructiveness of true mania. “He doesn’t register reversals and defeats and hard feelings the way other people do,” says David Klafter, a former colleague.

I ask Ackman about the condition while he is driving in a car with his family. He hasn’t heard of it, but says he is an “extremely resilient person.”

His 11-year-old daughter playfully chides from the backseat, “And you’re modest.”

Ackman is an activist investor, a respectable term for people who in the 1980s were known as corporate raiders. He buys big stakes in companies and then offers his opinions—loudly—on how to improve their operations. Often, Ackman has been a contrarian. He bought shares of Rockefeller Center when Manhattan real estate was on its back in the mid-1990s, and he launched an attack in 2002 on MBIA Inc., the powerful and politically connected bond insurer, when everyone else on Wall Street was convinced the company was gold-plated. In early 2007, he sounded one of the most prescient warnings about the credit bubble and the leveraged complex of American finance.

William A. Ackman, who turns 43 this month, has had the seminal financial career of the past two decades, which is to say that he’s had the seminal American career of the era. Almost immediately after business school, he started a hedge fund to manage millions for wealthy people—with no investing track record. About a decade later, he was forced to shut down. He endured regulatory investigations played out in the klieg lights of the press. He relaunched and clawed his way back to respectability, becoming a member of a new generation of Wall Street wise men. No hedge fund manager or investment banker will be able to replicate his trajectory for at least a generation.

Now he’s gearing up for one of the biggest battles of his professional life. After losing nearly $2 billion in a calamitous bet on the retailer Target Corp.—almost all that investors had given him for the investment—he is waging a proxy fight against the company. He will have a tough sell in the leadup to the annual shareholder meeting in May. Taking on a company as big as Target is almost unheard of. Target decries the contest as “costly and disruptive.”

Investors don’t want to hear much from hedge funds these days, and the tide may be turning against activism.

Panicked companies are focusing on their core business, not their capital structure.

At the same time, Ackman is talking about a much bigger turnaround situation: the United States. On a recent day in his glass-walled corner office in midtown Manhattan, he tells me with a smile, “I’m long America!” His tie is slightly loosened, and the sleeves on his blue shirt are rolled up. He is crafting a “plan to save the universe,” he says, with a slight glint that shows he is aware of the hyperbole. He recounts how he and Michael Porter, a Harvard Business School professor, recently had a “fantastic” meeting with Lawrence Summers, the director of President Barack Obama’s National Economic Council, to pitch their proposals for fixing the financial crisis and improving the market for mortgages.

“I’m long-term bullish on America but not on things turning around in the next few months, or even 12 months,” he says. “We’ve had the equivalent of a heart attack, but now we are in recovery, hopefully. It takes time to heal.”

These days, the public, enraged at the moneyed class of Wall Street operators, is in revolt over bonuses and rewards for failure, while Washington plans new regulations for hedge funds, and investors pull their money out of the industry. Ackman, who has been publicly vilified, can’t keep himself away from the spotlight. It’s almost in his nature to stand up and say that he has answers.

Overconfidence from financial types is what caused this grave economic crisis in the first place, of course. It can be a worrisome quality. But if you are Bill Ackman, you’re betting that confidence, correctly administered, might just get us out of it too.

Though often perceived as arrogant, Ackman up close might be the most winning salesman on Wall Street. Partly it’s because he explains each burst of an idea with overwhelming detail, lucidly laid out. But it also has to do with his boyish face—a rounded-off nose and high, rosy cheeks topped incongruously with a signature shock of gray hair that he’s had since high school. Going prematurely gray builds character, Ackman says.

“He’s gained a huge following of admirers, both male and female,” says Laurel Touby, founder of the internet company MediaBistro.com, which Ackman helped bankroll. “People fall in love with him. It’s almost like he’s the Bill Clinton of finance.”