The Labour leader claimed it would remove the influence of "big money" on politics and urged other party leaders "to grasp the nettle".
However, he made it clear that though donations from the unions would be capped he would not sever the flow of cash from individual union members through an annual political levy.
Mr Miliband said the loss of cash from the unions would be "painful" but challenged the other parties to offer compromises.
Capping the amount of money donated to Labour would rule out millions of pounds donated to the party by the unions, but it would also hit all other parties including the SNP.
Over the last year the Nationalists have benefited from a £1 million donation from Euromillions winners Colin and Christine Weir and a legacy of almost £1m from the late national poet Edwin Morgan.
Bus tycoon Sir Brian Souter has also given hundreds of thousands of pounds to the party over several years including a pledge last year of up to £500,000 to bankroll its Holyrood election campaign.
The application of a cap on trade union donations has been one of the sticking points in previous negotiations over reform of political party funding.
The Conservatives say Labour relies too heavily on money from affiliated unions such as Unite and Unison while Labour argues that big business provides too much of Tory party funding.
Mr Miliband said: "Let's take the big money out of politics. I hope Nick Clegg and David Cameron will come forward with their own proposals that say 'we're willing to take a bit of pain too, we are willing to make changes which will make things harder for our political party but it's in the interests of our democracy'.
"Let's make a concerted effort now to really grasp that nettle. I don't know how quickly we can get that reform. It's taken a long time to get to this stage, let's try and get the reforms moving."
The cap suggested by Mr Miliband is half the £10,000 proposed by the Parliamentary Standards Commissioner, Sir Christopher Kelly.
Mr Cameron, who has been embarrassed by the cash for influence scandal involving senior fundraiser Peter Cruddas who was taped apparently offering access to the Prime Minister in exchange for £250,000 donations, wants to set the cap on individual gifts at £50,000.
Mr Miliband said he accepted the reduction in funding could not be plugged by any increase in taxpayer funding in the current economic climate. He suggested limits on party spending should be set "substantially lower" in a bid to reduce the need for parties to seek huge sums from wealthy donors.
Unite welcomed the proposal saying it would help to restore faith in politics and it was pleased "the vital link between Labour and millions of working people is valued and will be retained".
The Tories dismissed the initiative as "virtually meaningless". Co-chairman Baroness Warsi said: "The Conservative Party has long argued that we need to take big money out of politics. That's why we proposed a cap that would apply equally to individuals, businesses and trade unions.
"But Ed Miliband wants an exemption for the union pay-masters who got him elected. His so-called cap would hit just 1% of Labour's trade union donations.
Labour described that as "absolute nonsense" and said that overall it would affect around 50% of donations.
SNP business convener Derek Mackay said: "The London parties still seem intent on protecting their main funding streams ahead of tri-party talks behind closed doors at Westminster. It's telling Mr Miliband said nothing about accepting a cap on trade union affiliation fees."
LibDem spokesman Simon Hughes said questions remained on Labour's commitment to real reform: "Why does Ed Miliband disagree with the Kelly proposals that people should have the freedom to opt in to donating to the Labour Party rather than the complex system of opting out?"