But his opponents continued to press for explanations, and a new opinion poll - taken before the scandal took off - showed Mr Balladur trailing the Socialist candidate in the first round, though still a clear winner in the second.
The government's strategy is to present the case as nothing more than police incompetence. The charge against the police is that in submitting their application for the phone of Dr Jean-Pierre Marchal to be tapped as a matter of urgency, the local police department of the Hauts-de-Seine region had not done enough to justify the request, putting it on the wrong side of the law.
This inadequate request was then forwarded to the head of the judicial police - Jacques Franquet - who forwarded it to ministers.
Mr Franquet's resignation was accepted yesterday by the Interior Minister, Charles Pasqua, who had rushed back overnight from a campaign trip to Marseilles. Afterwards, Mr Balladur's spokesman said that with Mr Franquet's resignation, "the matter should now be considered closed". There was widespread speculation, however, that the police chief was pushed by Mr Balladur to save the head of Mr Pasqua.
The question now is whether the scandal will stop here. Mr Balladur insisted throughout the day, in person and through his spokesman, that he had full confidence in the Interior Minister and that he would remain in his post. Like Mr Balladur, Mr Pasqua does not deny authorising the taps that were ruled to be illegal, but both insist that procedures were scrupulously followed.
The chairman of the independent commission that oversees surveillance, Paul Bouchet, confirmed yesterday that the procedures as such had been observed. The committee's judgement, though, was that the original reason for authorising the tap was illegal. Speaking at a news conference, Mr Bouchet said the request for immediate surveillance had given a reason that was not valid and had failed to state the "political and judicial context".
The request to tap Dr Marchal's phone was justified by reference to "blackmail and extortion with menaces by organised bands" - a legitimate reason in French law. But, said Mr Bouchet, there was no organised band, and certainly no organised crime on the grand scale which was what the spirit of the law presupposed. Nor was there, he said, any special urgency that warranted the use of special accelerated procedures. Worst of all, the police request had failed to mention that the subject of the proposed tap was related to the judge in a case that involved the misuse of funds by the governing RPR party.
Mr Bouchet declined to draw any political conclusions, saying that this was a matter for the politicians. But there was evidence that Mr Pasqua's star had fallen a little, not least in his tightlipped silence throughout the day.
The affair has reportly led to a cooling in relations with Mr Balladur, to the point where Mr Pasqua may no longer be sure of obtaining the post of prime minister in the event of Mr Balladur being elected president.
But Mr Balladur cannot afford to break with Mr Pasqua. The Interior Minister - with his hard line on terrorism, illegal immigration and law and order - gives his campaign credibility with the political right.
Last week, Mr Pasqua's speech at Mr Balladur's opening campaign rally was cheered and applauded, almost upstaging the Prime Minister. If Mr Balladur had to sacrifice Mr Pasqua, he would probably be sacrificing his election ambitions.
The Socialist presidential candidate, Lionel Jospin, drew attention yesterday to the fact that the Hauts-de-Seine dpartment was Mr Pasqua's home region. If Mr Pasqua's involvement can be traced back further than his statutory duty as Interior Minister, the affair has life in it yet.Reuse content