Much ink has flowed over the Erasmus Commission of Inquiry, but what few commentators have grasped is this: on Monday, when the Cape High Court ruled that the Commission was unlawful and unconstitutional, South Africa passed a litmus test of constitutional democracy.

What is that test?

It is when the ruling party's efforts to defeat its political opponents through means other than the ballot box are rejected by those institutions which the Constitution entrusts to limit - and prevent - the abuse of state power.

As the High Court found, the Erasmus Commission was nothing but an illegal attempt by the ANC provincial government to embarrass and discredit the DA. The ANC's strategy was two-fold:

Firstly, it was an attempt to divide the multi-party coalition in Cape Town so that it could take back the City. This became clear in May when Jacob Zuma said:

"We should not allow anyone to govern our city [ Cape Town ] when we are ruling the country." The ANC has never accepted its defeat in Cape Town ; the Erasmus Commission was simply another (and the most sophisticated) attempt to unseat the DA outside of the electoral process.

Secondly, the establishment of the Erasmus Commission was part of a strategy to prevent the DA from winning the Western Cape election next year. The ANC believed that if it could smear the DA by giving a host of aggrieved and dubious characters a public platform (under the guise of judicial impartiality), it might be able to retain the Western Cape despite the odds that are increasingly stacked against it.

The former Premier Ebrahim Rasool knew that executing this strategy on behalf of the ANC would be his best chance of surviving the purge of Mbeki allies that was always imminent when the political sands in the ANC had begun to shift in favour of Jacob Zuma. Of course, this did not work; once the ANC realised that the Erasmus Commission would never withstand the scrutiny of the courts, Rasool was removed from office and redeployed.

I said this week that the Court's verdict is not a victory for one political party or even the City of Cape Town alone. It is a victory for the Constitution. It shows that the judiciary is prepared and willing to check the ruling party when it abuses state power in an attempt to deprive its political opponents of power.

Because the DA and I (with the Desai Commission indelibly etched in our memories), knew that the Erasmus Commission was nothing but an unconstitutional smear campaign, we opposed it implacably. We knew that allowing the ANC to abuse power in this way would set a dangerous, and perhaps even irrevocable, precedent for our constitutional democracy.

The allegations against us of illegal spying were a fabrication from the start to give a cloak of legitimacy to all the illegal and unconstitutional activities that followed. So too was the falsehood that the City Council had paid for an investigation undertaken by the DA.

I welcomed the police investigation into these allegations and called for us to be charged in a court of law if there was any evidence of wrongdoing. This never materialised; instead the Premier chose to bypass the police, the prosecuting authority and the courts in order to set up his own political commission.

We made all these points at the time, but they were understood only by a handful of commentators and journalists. The majority gleefully seized on the possibility that it was the DA for once, and not the ANC, that might have been involved in wrongdoing.

There were three main criticisms levelled at us when we decided to fight the establishment of the Commission.

Firstly, several columnists, leader writers and satirists accused the DA, the City of Cape Town , and me personally of rejecting the Erasmus Commission because we wanted to conceal the truth.

In The Star, Jovial Rantao said: "What has Helen Zille got to hide? What is it that the Democratic Alliance is desperately trying to keep away from the public? What is it that the City of Cape Town , run by the DA, is trying to hide from the ratepayers?"

Pierre de Vos, a Professor of Constitutional Law, no less, declared: "One would have thought that if the DA - and Zille in particular - had nothing to hide, it would welcome the chance to clear its name through such a commission of inquiry".

That the City had asked Advocate Josie Jordaan SC to conduct an exhaustive inquiry into whether the City had broken any law in carrying out its investigation into Councillor Chaaban was lost on them. So, too, was the fact that the City had opened up all its files to the police to investigate, and withheld nothing. Despite a full and protracted police investigation, no evidence emerged that the City of Cape Town had contravened any law, or that the DA had engaged in illegal spying. No charges were laid.

Even the interim report of the Erasmus Commission found that neither the City nor the DA had engaged in illegal spying. The great irony is that the only person who was illegally spied upon was me. My cellphone records were illegally subpoenaed from Vodacom by the Erasmus Commission and handed to the ANC. It was my home phone that was found to be tapped at the time of the Erasmus Commission.

The simple reason for resisting the Erasmus Commission was because, as far as we were concerned, in the absence of any charges or evidence to the contrary, we had done nothing wrong. Why would we allow an unconstitutional political hit squad set up by the ANC to smear us through lies and innuendo that would be reported every day in the media as if they were fact?

Secondly, I was roundly condemned for the comment I made on radio that, "some judges allow themselves to be abused, and unfortunately Nathan Erasmus is one of them".

At the time of my statement, the Cape Argus claimed that "...Zille has resorted to...a ploy that...threatens to undermine the judiciary". The Sunday Times suggested that my statement contained "the dangerous accusation that a member of the judiciary is serving the interests of a political party". Professor De Vos argued that my comment was "no different from the ANC Youth League and Young Communist League diatribes and shows a scandalous disregard for our Constitution".

What these commentators never grasped was that the Commission was not a court and its chairman was not acting in his capacity as a judge; therefore he could not expect the deference due to a judicial process. This view has now been vindicated in the Cape High Court . Yet, as recently as Wednesday, an editorial in Die Burger stated that "although Zille is right [to have criticised Erasmus], she is also wrong". The implication is that somehow my statement about Erasmus constituted an attack on the independence of the judiciary.

In fact, it was Erasmus's acceptance of an appointment to a political commission that undermined the independence of the judiciary, not my criticism of him for doing so. In choosing to ignore the Constitutional Court guidelines that "judges should not get entangled in matters of political controversy", Erasmus opened himself up to legitimate criticism.

The court ruling itself notes that it was inappropriate for a judge to head a commission of inquiry which was so clearly political in nature, as this risked compromising the independence of the judiciary. It states: "That the government would want to use judges for their purposes is one matter but that judges should allow themselves to be used is quite a different one... The notion of being used by the executive in this way is anathema to the judicial calling and is the very antithesis of the separation of powers".

The third line of attack against us was that, by resisting the Erasmus Commission, I was becoming bogged down in a local issue. The Cape Argus accused me of suffering from "anxiety attacks" and asked: "Were those critics right who suggested that it would be problematic for one person to be both mayor and leader of the DA?"

Those who argued that I was allowing a parochial issue to distract me from more pressing national matters missed the fundamental point that the Erasmus Commission would have profound national repercussions if it were allowed to continue. If the ANC could get away with this kind of unconstitutional action, why would it ever have to accept the result of an election that went against it anywhere?

We fought the Commission because it was power abuse and because we agree with the old cliché that "bad things happen when good men do nothing." Power abuse - particularly at a time when the ANC is making menacing sounds about the Constitution - must be stopped in its tracks wherever it occurs.

There are some who argue that it is hypocritical of the DA to have opposed the Erasmus Commission but not the Zuma trial. They draw parallels with the Erasmus Commission and what they believe is the "selective prosecution" - a form of power abuse - by one ANC faction to discredit another.

The key difference is that Zuma is before a court of law; the DA and the City were not. The parallel would only have credence if a police investigation had failed to provide the basis of a credible charge against Zuma in court, only for President Mbeki to sidestep the criminal justice system and set up an illegal commission anyway, handpicking his own judge to chair it, in order to conduct a political witch-hunt to discredit Zuma. If this had happened, it would have been credible to talk about persecution. However it did not. Zuma is being properly prosecuted through the criminal justice system.

As we move forward, we can be encouraged that we have a Constitution and a judiciary that effectively protect us from power abuse by the ANC. The High Court ruling marks a watershed judgment on the separation of powers and the prevention of power abuse by the ruling party. At a time when the ANC often seems to disregard the constitutional constraints on its power, the fact that the courts have acted decisively and emphatically is something from which we can all take heart.

This article by Helen Zille, leader of the Democratic Alliance, first appeared in SA Today September 5 2008