Roll up, roll up, come and jump on the 2015 general election bandwagon. It helps if you are rich and like the Tories, but it matters little whether you were born in Britain or insist on keeping your donation private. There are people waiting at glitzy fundraising dinners right now to take your cash and distribute it with laser-guided precision to the candidates that most desperately need it – those fighting to win the tightest constituencies in the land.

An analysis produced for the Observer by the Bureau of Investigative Journalism reveals that hundreds of thousands of pounds have been funnelled to Tory candidates ahead of next year's election, one that pollsters suggest may produce the tightest result in living memory.

With Labour having to rely largely on the unions for funding, the Tories appear much more fleet-footed and energetic in their pursuit of donations. The bureau's analysis shows that leading Tory hopefuls are powering ahead in terms of the sums being raised – much of it via obscure clubs and organisations legitimately used to mask the identity of donors.

The issue of donations has been thrown into sharp relief following the Tories' annual fundraising dinner, held last week at the exclusive Hurlingham Club in south-west London. The event proved a huge money-spinner with oligarchs, Middle Eastern businessmen and City financiers vying to bid huge sums at an auction that raised £500,000 for David Cameron's party. At this year's dinner a Russian banker – the wife of a Putin ally – paid £160,000 to play tennis with Boris Johnson and Cameron. A bottle of champagne signed by Margaret Thatcher went for £45,000.

The event bordered on the surreal, according to one person who attended. Bids were made using "spotters" who patrolled the event wearing pouches, from which they sold £50 raffle tickets, and carrying wands that lit up when someone joined the auction, lending a theatrical feel to the event.

The total sum raised from this year's event – as with the definitive guest list – is currently unknown. But last year's fundraiser, attended by 449 guests who sat at tables costing up to £12,000 each, saw £1.1m in donations to the party in the week following the event, according to filings registered with the Electoral Commission.

Much of this money, it appears, is now finding its way to the marginal seats which have hitherto benefited from the largesse of former Tory party treasurer Lord Ashcroft, who has given the party millions of pounds down the years but has now turned off the tap, according to his spokesman.

The bureau's analysis shows that, since the last election, some £321,182 has been funnelled to Tory candidates contesting the 20 most marginal seats in Britain – compared with just £74,545 to their Labour rivals. The Lib Dems have spent more than £183,000 across the same 20 seats – their figures skewed by the fact some £84,000 has been ploughed into fighting Dorset Mid and Poole North, identified by the party as a crucial must-win constituency.

Beyond the top 20 tightest marginals, the Tories are also doing well in the fundraising stakes. David Warburton, who will contest the Lib Dem seat of Somerton and Frome, has received almost £60,000 since 2010. Anne-Marie Trevelyan, fighting Berwick-upon-Tweed, another Lib Dem seat, has received almost £50,000. Michelle Donelan, bidding to take the Lib Dem seat of Chippenham, has received £43,000 in donations, and Rebecca Pow, targeting the Lib Dem seat of Taunton Deane, almost £25,000.

According to Electoral Commission records, Alexander Temerko, a Russian businessman with offshore wind interests in the north-east, who paid £90,000 for a bust of David Cameron at last year's Tory party fundraiser, has donated almost £9,000 in campaign materials to Trevelyan and has also backed Tory MPs Michael Ellis, James Wharton and Guto Bebb, all of whom command only slim majorities.

A large portion of donations to Tory candidates fighting marginal seats is coming from the United & Cecil Club, an obscure organisation which paid for two tables at last week's fundraiser. In November, the Guardian reported that the Conservative party had received a total of £1,543,330 from the United and Cecil Club and another dining club, the Carlton Club, since 2001 – of which £1,248,125 had been donated since Cameron became leader.

Now it appears that the U&C is becoming one of the main vehicles used for distributing cash to Tory marginals. Between 2010 and the first quarter of this year, it was the biggest funder of Tory candidates in the 10 most marginal seats in the country. In all, U&C has given almost £285,000. Almost half of its donations were made this year alone.

Targeting marginals will help determine who wins the next election, according to experts. A poll carried out for Ashcroft across 26 of the most closely contested constituencies in May suggested that there had been a 6.5% swing from the Conservatives to Labour. If repeated nationally this would be enough to topple 83 Tory MPs and give Ed Miliband a majority, according to Ashcroft. But, as he wrote a couple of months ago on his website, "half of voters say they may change their mind before the election – and there is still a year to go".

Additional reporting by Victoria Parsons and Gaby Bissett