Charlie Whelan, Unite: Ashcroft comparison "is actually very insulting"
Gordon Brown's former spokesman has said it is "insulting" to compare union support for the Labour party with Lord Ashcroft's funding for the Tories.
Charlie Whelan, now political director of the Unite union, told the BBC funds came from "ordinary working people" who wanted Labour to win the next election.
He attacked Tory donor Lord Ashcroft, who has non-domiciled tax status.
Unite's key role in the British Airways strike has opened the government up to criticism from opposition parties.
Mr Whelan worked for Mr Brown during the 1990s but was forced to resign in 1999 when he was linked to leaks surrounding the resignation of cabinet minister Peter Mandelson over a home loan.
He told the BBC's political editor Nick Robinson it was no secret that "as a union we want a Labour government".
He said: "Not because the union wants it, but because millions of working people want it.
"What we as a union, Unite, are trying to do is making sure we have a Labour government by talking to our members and explaining it to them."
He said he was coordinating efforts by the union to contact members, especially in key marginal seats, to encourage them to vote Labour.
Asked whether he was "Labour's Lord Ashcroft", he replied: "It's not funny; it's insulting.
"It's insulting to the millions of working people who give a small proportion of their union dues towards the Labour Party, to support the party they founded."
The Conservatives have claimed Mr Whelan is "inside Number 10" directing Labour policy - and its election campaign - by controlling where donations are spent.
But he said: "I don't wield any amounts of money. What I do is make sure that the members who pay money to Unite, their money is spent wisely.
"Every single penny we spend is open, above board and for you to find out."
Mr Whelan said Unite was helping to find candidates to stand for Labour at the general election.
He said: "There's no secret that a party that represents working people wants working people in Parliament."
Unite has given £11m to Labour over the last three years. Lord Ashcroft's donations to the Tories amount to about £4m in recent years.
Much of the peer's money has also been targeted at marginal seats.
Lord Ashcroft admitted earlier this month that he was non-domiciled in the UK, meaning he does not pay full UK tax on his overseas earnings.