DES MOINES, Iowa (AP) _ Tom Harkin discarded his jacket and tossed out a challenge to President Bush that had his audience of autoworkers cheering wildly.

''Ol' George,'' he said, smiling broadly. ''He's never met anybody like me. We're going to have some fun.''

Fun is not a notion ordinarily associated with Harkin. Lean and intense, the Iowa senator and would-be president has been a political lightning rod since his days as a brash young congressional aide 20 years ago.

Through 10 years in the U.S. House and two winning campaigns for the Senate, he's won the grudging respect and admiration - if not affection - of friends and foes alike. His political toughness is legend in the state.

Now Harkin, one of the Senate's most outspoken liberals, wants to bring those traits to bear in the White House. He's running for president hoping to turn the nation's attention from events abroad to its problems at home.

He assaults the ''gluttons of privilege'' symbolized by ''George Herbert Walker Bush'' and ''J. Danforth Quayle.''

''It's time for us to quit being a party of accommodation to a Republican agenda,'' he says. ''I believe there's a hunger in America, a hunger to turn away from the policies of the 1980s.''

Harkin's headstrong style has helped him survive politically while bringing a liberal message to an electorate many argue is increasingly conservative.

''He's tough, he's hard-hitting, he's smart, he's a street-fighter,'' said former Iowa Democratic Chairman Ed Campbell.

''Tenacity is certainly one of his strengths,'' allows Iowa Republican Chairman Richard Schwarm.

Harkin himself offers a succinct description of his chosen career.

''Politics,'' he says, ''is a contact sport. Never defend, always attack.''

But he also says of himself, ''I'm really a very gentle person and a lot of the causes I've worked for have been gentle causes.''

A longtime champion of the disabled whose own brother is deaf, Harkin looks on passage of the Americans with Disabilities Act last year as one of his greatest legislative accomplishments.

Harkin, 51, who entered the Democratic presidential race on Sunday, is a product of hardscrabble southern Iowa coal country.

He grew up in a two-bedroom home, one of six children born to a mother who died during his childhood and left a coal miner father sick with black lung disease.

Schoolmates remember a quiet youngster from the wrong side of the tracks, taken under the wings of family friends and shipped to Catholic schools in Des Moines.

''Most of the time he was very serious, like very intense on being sure that he did the things he was supposed to do,'' said high school classmate John Dorrian. ''It was a very strict school where discipline was handed out. You were there to get an education.''

''In Des Moines, there's a side of town that's considered more elite,'' said Kathie Lyman, a high school girlfriend. ''None of us had that. It tends to develop your character.''

Harkin doesn't spend much time thinking about the why of it all.

''I'm not a real introspective person,'' he said. ''I don't sit down, perhaps like Mario Cuomo, and try to examine why I believe this, and what is the meaning of this or that. I'm not really that introspective. I understand on my gut level.''

Whatever the genesis, the result is a Midwestern populism that traces its lineage to people like William Jennings Bryan. It's expressed in white-hot - and occasionally earthy - rhetoric about those who have and those who don't.

Harkin is a frequent and vocal critic of Bush administration policies, particularly on agriculture and foreign affairs. Earlier this year he pushed a reluctant Senate into voting on whether to authorize the use of military force in the Persian Gulf - something he opposed. He advocates deep cuts in defense spending and shifting the money into roads, bridges, schools and other domestic needs.

From high school on, Harkin's path was a straight line: Iowa State University on an ROTC scholarship, eight years as a Navy pilot, then on to the staff of Iowa's U.S. Rep. Neal Smith, and his first brush with controversy.

Assisting a congressional delegation on a visit to South Vietnam, Harkin photographed notorious ''tiger cages'' where political prisoners were being held. Unable to get much action from Congress, he sold the pictures to Life Magazine, causing an uproar that ended his career as a staffer.

During that stretch, Harkin and his wife, Ruth, were finishing law degrees at Catholic University's night school. They returned to Iowa, where she was elected a county prosecutor and he went to work as a Legal Aid lawyer.

Ruth Harkin, now with a high-powered Washington law and lobbying firm, may be her husband's most trusted political adviser. On the day he decided to run for president, she was the last person he spoke with before making up his mind.

Mindful of his own childhood largely without parents and of the rigors of a national campaign, Harkin withdrew on a family vacation before deciding whether to enter the race. Daughters Amy and Jennifer - ages 9 and 15 - won't be a fixture on the campaign trail, he said.

''There are some things more important than being president - taking care of your family, being a responsible parent, dutiful husband,'' he says.

The tiger-cage flap that cost Harkin his congressional job also served to launch his political career. Harkin was warned he would never again work in Congress, so he set out to earn a House seat of his own.

In 1972, he launched an upstart bid against veteran GOP Rep. Bill Scherle in an overwhelmingly Republican rural district. He lost soundly - but didn't give up.

''He never quit knocking on doors,'' said Jim Riordan, who was Harkin's driver during his early days in Congress. ''The guy just took a couple of weeks off and then plunged right back in and started campaigning.''

Harkin ousted Scherle on his next try, with a little help from the anti- Republican backlash against Watergate.

After 10 years in the House, he ran against incumbent Republican Sen. Roger Jepsen in 1984 and knocked him off easily, thanks to another piece of political luck. A few months before the election, it was disclosed that Jepsen had visited a notorious Des Moines sex club.

After fighting off a 1990 challenge from Republican Rep. Tom Tauke, Harkin turned to a bigger stage. He began traveling the country, in his words, ''to see how it resonates, both the message and the messenger.''

Some had few doubts about the outcome.

''I can honestly say as far back as 1972, I knew this man was going to try to run for president someday,'' said Riordan. ''I could just see that ambition was in him.''

Harkin is more pragmatic about his political path.

''I'm not a believer in destiny or anything like that,'' he said. ''Everything that's happened to me this year is just like one more nudge.''