Legislate in haste; repent at leisure. When Labour passed the Political Parties, Elections and Referendums Act 2000, it was meant to be the final word on sleaze. The law was to be zero tolerance, strict liability, and would clean up politics once and for all. No-one was to escape the net. But they never expected the net to catch an entire shoal of Labour fish wriggling in disbelief.

Following Peter Hain's departure from office, to help police with their inquiries, there are a number of prominent Labour figures potentially in the dock.

The elusive northern property developer David Abrahams, who connived with Labour's former general secretary Peter Watt to set up a network of illegal proxy donors to channel £600,000 in illegal donations to Labour politicians.

Our own Wendy Alexander's case is with the Electoral Commission, and the deputy leader of the Labour Party, Harriet Harman, also accepted an impermissible donation. Questions have been raised about the reported £300,000 donation to Labour from Glasgow-based businessman Imran Khand, through a front organisation, the Muslim Friends of Labour.

No doubt there will be others. The mystery is why Labour politicians were so ignorant of their own law. The Electoral Commission assigned £180,000 for education programmes to ensure party officers and elected members would be in no doubt of their obligation to obey the letter of it. Not money well spent, it seems.

But Labour politicians are incensed. Suddenly they've realised that no-one is safe from this terminator statute. Politicians aren't accountants, after all, and most of them are as uncomfortable with figures as the rest of us.

You can sympathise - well, a little. Anyone who has ever been under investigation by the Revenue or VAT inspectors, or had their expenses examined, knows accounts are rarely glitch free. You can be incriminated by any number of minor infractions you weren't aware of. And, of course, for agencies such as the Revenue, ignorance is not a defence.

But as fallen politicians plead workload or plain incompetence, they're being watched by an electorate whose patience is wearing thin. Why should MPs get off because they "didnae ken", when that defence doesn't work for ordinary people? The trouble is politicians really don't see themselves as ordinary people. There is a presumption of collective innocence - they really don't think the rules apply to them.

It's always "other people" - corrupt businessmen, dodgy lobbyists - who are the targets of anti-sleaze laws, not their own kind. Most become indignant at the mere suggestion they could break the law. And if found bang to rights, they attack the law itself - it was "badly drafted", "wrongly interpreted", the law enforcement agencies were "politically motivated", "the other parties are just as bad ..." etc.

Why should busy politicians have to give up their careers just because some underling didn't register campaign donations on time? Or because some numpty solicited a donation for an impermissible donor and tried to cover it up? Or had set up a network of fictitious donors to disguise the scale of contribution from businessmen? As Wendy Alexander has said, there was "no intentional wrongdoing" so why should they be treated as criminals?

The Electoral Commission - a Labour creation - now bears an onerous responsibility. It has become the guardian of political probity, and holds in its hands the very future of the government. If it decides this week that other Labour figures should, like Hain, have their cases referred to the boys in blue, then the damage to Gordon Brown's administration will be incalculable. If his Scottish leader, his UK deputy leader, a slew of senior office-bearers and others we don't even know about are forced to resign, then it would be curtains.

Mind you, Alexander's team are now saying that even if the Electoral Commission finds against her, she will not resign. They accept the law was breached when she accepted a £950 donation from tax-exile businessman Paul Green, but insist she knew nothing of it, even though she had sent a letter of thanks to his Jersey address. Her campaign team claimed the donation had not come from Mr Green but from a UK-based company, Combined Property Services. This was untrue.

But it was, we are assured, a muddle not a fiddle. Why should she be faced with resignation for a mistake she didn't make over a few quid? Well, the problem here is the law is tightly drawn, and the actual sum involved doesn't come into it. As the "regulated donee", Alexander is responsible for everything that happens in her name. And, despite her defiance, even a mild censure from the Electoral Commission might be enough to end her career.

What the commission will have to do this week - if it exonerates Alexander - is explain why, when the law has been broken by the admission of the parties involved, the case should NOT be referred to the relevant authorities. In short, why the police and prosecuting authorities shouldn't at least take a look at it. After all, the Electoral Commission is not judge and jury, but a regulator. It is supposed to refer any cases where there is a prima facie breach of the law over to the agencies equipped to assess them.

The danger here is that if the commission doesn't make a convincing case for leniency it will undermine its own credibility as well as Alexander's. The public will lose trust in the commission and whatever trust they still have left in politicians.

Could Alexander tough it out if that happened? She won't "walk away from her reputation", we are told. But what reputation would she have left? The prime minister himself said that Hain had done the "honourable" thing in stepping down to clear his name. Would Alexander be able to argue that her infraction is so minor she can remain honourably in post? The court of public opinion will have to judge that.

I wouldn't like to be the legal eagle in the Electoral Commission who has to adjudicate on the Alexander episode - it looks like a no-win situation. If she's exonerated, a lot of people will claim a cover-up; if they call in the law, it's probably the end of her career. Either way, the very foundations of this government are shaking - and we'll learn in a few days whether Labour's own law is going to be Alexander's undoing.