SOUTH DAKOTA, where life grinds at the pace of giant tractors labouring over heavy fields, is the unlikely starlet of this season’s election drama.

The political elite has flocked to fawn on its rugged stage, lavishing money, gifts and attention on this hesitant power-broker. President Bush can’t stay away from its snow-crusted prairie and frosted towns. He has campaigned four times in a state with 450,000 voters, and will squeeze in a fifth visit tomorrow. His three predecessors visited nine times in two decades.

In the past five days Dick Cheney, the Vice-President, his wife Lynne, and Laura Bush, the First Lady, have all also tried in person to break the deadlock of reserved South Dakotans.

The reason is simple. Any analysis of the quirks and undercurrents of this year’s congressional elections comes to the same conclusion: South Dakota is the key to controlling the Senate. In the cluster of must-win contests, the fifth-least populous state in the nation comes top. It has been seen as the tightest race on the map for months.

It is possible for either Democrats or Republicans to lose here and still run the Senate by making gains elsewhere, but that is unlikely. So the 22 voters gathered in a public library to meet Tim Johnson, the incumbent Democrat, hold extraordinary power in their hands. Their votes will help to decide the balance of power for the next two years, colouring the short-term political agenda and the run-up to Mr Bush’s re-election bid.

The opportunity has become a burden to many South Dakotans. Identified early as a key state, both parties have exploited cheap airtime to bombard voters with negative advertising. The two candidates have run an estimated 20,000 commercials since January, not including those aired by the national parties and outside interest groups.

The breathless campaigning has left Mr Johnson and his Republican challenger, John Thune, intense, clean-cut men, locked in an exhausting dance of death as they try to club each other into submission.

Much of the nastiness is imported, an ingredient in a wider proxy war between the top Democrat and top Republican in the land. South Dakota’s senior Senator, not on the ballot this year, is Tom Daschle, the Senate leader and obstructer-in-chief of Bush legislation.

The President hand-picked Mr Thune for the task of unseating Mr Daschle’s protégé, and Democrats are campaigning as if Mr Bush himself were on the ballot.

The campaign has consumed £14 million of party money, almost doubling South Dakota’s record, and equates to $30 (£19) a head.

The high stakes have led to mistakes. With the contest likely to be decided by a few hundred votes, Democrats in particular have been trying to wring support from every corner of the state. An attempt to sign up historically reluctant American Indians has ended in the courts amid claims that the names of dead people found their way on to the electoral register.

The campaigning tone appears misplaced in a state that actually likes its politicians and, more than most, votes for individuals above parties. Both Mr Bush and Mr Daschle are hugely popular in the state. The Senate opponents are also widely liked. Mr Thune represented the state in the House of Representatives, and at least a quarter of the state has voted for both him and Mr Johnson at one stage.

Mr Bush swept the state with 60 per cent in the 2000 presidential election, but in a state that receives more federal money than it pays in tax, populism counts as much as conservatism.

For most South Dakotans, the race boils down to this: will they get more out of Washington with Mr Daschle leading the Senate and Mr Johnson sitting on the crucial Senate budget committee, or with Mr Thune’s promise of a hotline to the White House? In the public library of Brookings, a junction town where grain silos tower over the main street, Mr Johnson stares at his feet awkwardly as he is assailed by a Muslim woman over Iraq. Unlike 1991, when he vigorously opposed the Gulf War, Mr Johnson backed Mr Bush’s request for authority to use force. He looks tired. The campaign has been gruelling and, possibly, largely fruitless. About 90 per cent of voters have known all year how they will vote, and the polls — with Mr Johnson ahead by a nose — have remained stuck.

Asked if he thought people would be choosing between the candidates themselves, or between Mr Bush and Mr Daschle, Mr Johnson says: “They are aware of the dynamic, but they will be voting for John or Tim.” For the country, and for Mr Bush in particular, the difference between John and Tim is enormous.

Candidates' conundrums

Whichever party triumphs on Tuesday, control of the Senate could change three or four times before the new Congress is sworn in next January. The Missouri election is, in effect, a by-election, meaning the winner takes office immediately. Were the Republicans to capture the seat they would enjoy a majority of one for the final two months of the old Congress.

However a Republican Senator from Alaska, Frank Murkowski, is standing for state governor. If he wins he will have to quit his Senate seat by December 5, giving the Democrats a majority. Five days later, as governor, he will be able to appoint a replacement, putting Republicans back in control. Jesse Ventura, Minnesota’s Governor, can appoint an immediate replacement for Paul Wellstone, the state’s Democratic Senator who died last week. He could appoint the winner of Tuesday’s contest, but has hinted that he might appoint a member of the public. As control swings between parties, the Senate must complete next year’s budget.

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