When I am dealing with a disagreement between children, when it is clear they are both in the wrong and both behaving in a way that is not acceptable, I let them know that the argument "I'm doing it because he is doing it" or "But he's doing it too", are simply not good enough reasons for doing something.
If it is wrong then it is wrong, whoever is doing it. So whilst I agree with Iain Dale that the Labour Party are showing stunning hypocrisy in criticising Lord Ashcroft's tax status, it is actually no defence for the Tory Party to say that they are doing it because Labour are doing it.
That was the argument many MPs made in making false expenses claims. It didn't wash then and it doesn't wash now.
There is, however, another subtly difference between labour and Tory tax dodgers making donations. The Labour people do not have a hand in the political strategy of the party. They make donations, but do not seek influence within Labour. Compare this to Lord Ashcroft who plays a major role in the Tories target seat strategy. This is a point the Tories choose to ignore when making comparisons.