The Better Together campaign faces calls from the SNP to return the cash donated by Ian Taylor, amid claims the businessman has links to Libya, Iraq and Serbia.
McLeish told Scotland on Sunday that Unionist leaders had to “respond to legitimate questions” about Taylor’s gift to Better Together – the campaign’s largest donation so far.
Taylor’s oil firm Vitol has insisted that the allegations made about them are inaccurate and a spokeswoman for the firm said they were taking legal advice on the claims.
SNP Westminster leader Angus Robertson accused Better Together of being “desperate to cling” on to Taylor’s cash, which made up nearly half of the campaign’s war chest of more than £1 million.
Robertson went on to attack Better Together leader Alistair Darling for refusing to return the donation and said the decision showed the former Chancellor’s “credibility is totally shredded”.
McLeish warned that the campaign against independence could be harmed by the donation, but would not say whether Darling should handed back Taylor’s cash.
He said: “Labour has to be clear about whether this is the type of finance we want in the campaign. We’ve got to make a judgement about whether we have an American situation, where lobbyists are targeting campaigns all the time and politics degenerates. We need absolute transparency. We have to be very careful about what kind of money should be used. The unionist campaign has got to respond to legitimate questions and the criticism that’s been put forward.”
McLeish has also attacked the row between Better Together and the SNP as an “exchange that treats the public with contempt” and could alienate voters ahead of the 2014 referendum.
Labour MP Darling defended Taylor’s donation, saying “people who care about Scotland should be able to speak out against the nationalists without facing these kind of personal attacks”. He said: “Ian Taylor is a respected figure in Scotland and in the UK. He has made a big contribution to Scottish life, including a personal investment which has revived the Harris Tweed Industry in the Outer Hebrides.”
The pro-independence Yes Scotland campaign has a war chest of £1,625,797, including £1m from Ayrshire lottery winners Colin and Christine Weir.