It's difficult now to recall the giddy sense of possibility that greeted Paul Wellstone's 1990 election to the U.S. Senate. Running against Minnesota Republican Rudy Boschwitz, a popular and seldom controversial incumbent with a $7 million war chest, he was widely considered the burnt offering of a state Democratic Party that had never really wanted him in the first place. Only weeks before Election Day, polls showed him running 16 points behind. Wellstone eventually triumphed by running a low-budget campaign that was risky, inventive, populist in tone, and unabashedly left-liberal. In so doing, he became the only candidate to unseat a Senate incumbent that year. If popular disgust for Beltway elites has become a matter of conventional wisdom in the decade since, it is easy to forget that Wellstone's improbable win was among the first portents compelling the discomfited hordes of Washington pundits and party leaders to admit there was trouble in the air.

Wellstone quickly made a name for himself - first by openly denouncing the racist politics of Jesse Helms and his kind, and soon thereafter by emerging as one of the most vociferous critics of the war in the Persian Gulf. In the latter capacity, he made the rounds of TV talk shows and staged a controversial, emotionally charged press conference in front of the Vietnam War memorial. He was the "Senator from the Left," exulted The Nation's David Corn. Mother Jones held him up as "the first 1960s radical elected to the US Senate." George Bush offered a more withering assessment: "Who is this chickenshit?" he muttered after being grilled by Wellstone at a reception for new members of Congress.

Wellstone showed limited opposition to the military action of a Republican president, but when it came to Bush's Democratic successor, Wellstone was a virtual cheerleader. He supported every single Clinton troop deployment, missile launch, and bomb drop: Somalia (1992), Haiti (1994), Bosnia (1995), Iraq (1998), and Kosovo (1999). This record makes a person wonder if Wellstone's principles aren't severely diluted by partisanship. Is his political philosophy as simple-minded as Democrat=good, Republican=bad? Maybe. Unlike Democrats and Republicans, Greens in the US Senate wouldn't have to continually compromise their instincts and principles in order to please their party leader in the White House.

In the Senate Wellstone was its most liberal member, as he liked to put it, "from the Democratic wing of the Democratic Party", who did not hesitate to accuse Bill Clinton of selling the party's soul. Soon after he was elected, he described Jesse Helms, the curmudgeonly Republican conservative from North Carolina, as "everything to me that is ugly and wrong and awful about politics".

Over the years, Wellstone would mellow. He led some notable bipartisan initiatives with ideological foes like Sam Brownback of Kansas, and was one of the most liked and respected members of the Senate club – "a decent, genuine guy", in the words of the former Republican majority leader Bob Dole. A tower of strength was Wellstone's wife Sheila, his high-school sweetheart and the other half of one of Congress's most admired domestic political partnerships – and who died with her husband and their daughter in a plane crash on Friday, on the way to a funeral.

Never did Wellstone lose the courage of his convictions. Hardly had he arrived in the Senate in 1991 than he was voting against the Gulf War to drive Saddam from Kuwait. This month, he was the only Senate Democrat facing a tough re-election fight to vote against authorizing President George W. Bush to use force against Saddam Hussein. Predictably, Wellstone's adopted Minnesotans were not bothered in the slightest.

Paul Wellstone, called the conscience of the US Senate for his passion and liberal convictions, was killed in an air crash yesterday, in the final days of a knife-edge mid-term election campaign.

Wellstone, 58, was seen as a symbol of the anti-war movement for voting against President George Bush earlier this month on the resolution authorizing the use of force against Iraq, a stand that gave this contest in Minnesota national significance.

He was killed with his wife and daughter, three campaign staff and two pilots when their small propeller plane went down in icy rain at Eveleth, in north-eastern Minnesota.

His death has shocked Minnesota, where he was seen as a defender of the voiceless and the environment, a man who gave a small state a higher than usual profile in Washington. It also threw into chaos a race seen as crucial to control of the Senate, where the Democrats hold a narrow majority.

By the time voting gets under way on November 5, each side in the race is expected to have spent upwards of $10m - about $2 for each Minnesota resident.

The race for the Minnesota Senate seat, which Wellstone held for 12 years - two terms - was perhaps the most closely followed in the country, not only because of its importance to the future of the house, but because Wellstone's stand on Iraq had been seen as courageous, but political suicide.

Mr. Bush personally intervened to anoint Wellstone's challenger, Norm Coleman, and has visited the state three times to campaign on his behalf.

Wellstone, a college professor who turned his lectures on grassroots organising and protest politics to good use, was the only senator facing re-election to oppose military action against Iraq.

The conventional wisdom in Washington was that would ensure his losing. "At one time even I felt that if I voted that way, I would certainly lose, but I don't feel that way now," he told the Guardian on Thursday.

In fact, he believed people would have lost respect for him had he taken a more expedient route. "I think people are being very respectful. I think it is more important to people that I rendered the right decision for me in spite of their politics."

The last hurrah of the ageing leader is an ancient motif of US politics. Even in normal circumstances the tears are never far away when a once-great figure goes down to his final defeat.

The brief, bizarre and tragedy-tinged closing chapter of Walter Mondale's career came to an end in a hotel ballroom just before 10am when he conceded defeat, with grace and good humour, to the brash and breezy Republican Norm Coleman, the new senator from Minnesota.

Then the 74-year-old former vice-president and presidential candidate shuffled off the stage, literally and figuratively, back to the comfortable oblivion he left less than two weeks ago.

By then the hall was deserted, except for the media and 100 or so remaining supporters, to whom this was the final moment of desolation.

Mr Mondale came out of retirement to take over the campaign when Paul Wellstone, the former senator and standard-bearer of the Democratic party's left, was killed in a plane crash.

Wellstone's adoring supporters were kept going through their grief by the adrenaline of battle and the determination to win in his memory, Now, at last, many of them broke down and hugged each other, silent except for their sobs.

At the end of a terrible fortnight for them, and a terrible night for their party, there was nothing anyone could say.

Mr Mondale spoke to young activists, who in Minnesota had been galvanised by Wellstone's uncomplicated and unfashionable sense of injustice.

"It's important for you to know that your ideals are tested more in defeat than victory," he said. "This is not the end but the beginning of what you can do. You will be needed now more than ever. You are the future. Stand up and keep fighting."

Fighting for what, though? During the long night, well before the reality of their own defeat had set in, there was a sense of futility developing among the Minnesotan Democrats. It can only have been echoed, if a little less passionately, elsewhere in the country.