Labour will today accuse the Tories of bankrolling their election campaign with "shady money" from secretive clubs and societies.
Deputy Labour leader Harriet Harman will highlight how donations to the Tory party from unincorporated associations have reached more than £5.5 million since 2010, with money from the groups being channelled to key target seats.
Labour analysis of Electoral Commission figures showed that donations from the organisations formed more than half the money raised in 27 of the key seats identified by the Tory party.
Ms Harman will also highlight a drop in membership income for the Tories in 14 of their 20 most marginal seats.
Unincorporated associations such as the United & Cecil Club are able to make donations to parties without having to publicly declare the individuals behind them.
Ahead of the speech in London, Ms Harman said: "While the Tories can't beat us on the number of activists campaigning on the ground in key seats -they have a steady stream of big donors willing to bankroll their campaign.
"The same people they handed a tax cut to. The people who go to David Cameron's dinners.
"Hedge funds, bankers, millionaires - you can see who they are. But there's one group of people bankrolling the Tories whose identity is completely secret.
"Since the last election, the Tories have been given over five and a half million pounds by unincorporated associations - organisations who don't declare where their money comes from. They donated £372,000 in the last three months alone.
"This money is being funnelled into the Tories' key marginal seats. In 27 of their key seats, more than half the money raised since 2010 has come from these secretive clubs.
"It's no wonder they need this shady money. In three-quarters of their most marginal seats, their membership is falling.
"We may be outspent but we won't be out-organised or out-campaigned. They want to buy the election. We're fighting to win it."
The Conservative Party raked in £7.2 million in donations in the second quarter of the year, almost double the Opposition's total.
Some £372,183 was from unincorporated associations, the Electoral Commission figures showed.
Labour's biggest donation in the second quarter of 2014 was £683,342 from Unison and the Tories have frequently attacked the extent of union funding for Ed Miliband's party.
A Conservative Party spokesman said: " It seems Labour, a party funded by union barons who pick the candidates and the leader, are spending more time trying to invent nonsense about the Conservatives than research and develop policies for the next election.
"If they spent less time playing politics and more creating some economic policies for the future the public might not hold Mr Miliband in such poor esteem."