The timing of Mary Harney's decision may have come as a surprise, but not the decision itself, writes Stephen Collins, Political Correspondent.
Mary Harney's decision to step down as leader of the Progressive Democrats after a hugely successful 13 years in the job has changed the dynamic of Irish politics at a critical time in the run-up to the next election.
A Government with Bertie Ahern as Taoiseach and Michael McDowell as Tánaiste would be a very different and more volatile arrangement than the current coalition, as both leaders weigh up their prospects going into the election.
Of course, it remains to be seen whether Mr McDowell will be the next leader. Whether or not he will face a challenger, he will be the clear favourite to take over the party he helped to found almost 21 years ago.
One thing is certain and that is that the very existence of the PDs will be on the line next time out. Whoever takes over as leader will be playing for the biggest stakes of all, sheer survival.
Two of the triumvirate that founded the PDs, Des O'Malley and Mary Harney, have served as leader and the odds must be that Mr McDowell will now succeed to the post. If that happens, politics will almost certainly become more exciting and unpredictable.
It was Mr McDowell who coined the phrase that in order to survive the PDs had to be either radical or redundant. With both published and private opinion polls now showing the party on the very edge of survival, it is clearly a time for a more radical approach.
In her nine years as Tánaiste, Ms Harney developed a relationship of trust with Bertie Ahern that enabled the Coalition to survive a series of potentially destructive political events. Both were experienced politicians who tried to avoid confrontation and they worked hard at keeping Coalition relations smooth.
Ms Harney was particularly keen to dispose of the image of the PDs as a party that looked for "heads on plates" whenever a crisis arose. Like Mr Ahern, she needed to demonstrate that her party was capable of coalition and she did this effectively.
During her first two topsy-turvey years as leader, after she succeeded Des O'Malley in October of 1993, no one would have predicted that Ms Harney would have served for nine uninterrupted years as Tánaiste and have a huge impact on the course of Government policy.
When she defeated Pat Cox to take the leadership, all looked rosy for the new leader and the party, which was then in opposition. However, things went badly wrong in her first year as Mr Cox left the PDs to fight Des O'Malley in the European election of 1994. It was a devastating trauma for Ms Harney and her party, which was made all the worse by the fact that Mr Cox won the battle. The PD civil war left wounds that have never healed.
Over the following two years she managed to claw her way back, despite the defection of Martin Cullen, after the departure of Mr Cox. Her stunning Dáil performances during the crisis which destroyed the Fianna Fáil - Labour government of Albert Reynolds in November of 1994 helped to restore her confidence and her political reputation.
However, her first election campaign as party leader in 1997 was an unmitigated disaster. The PDs, who had won 10 seats in 1992, dropped to just four in 1997 and as leader, Ms Harney had to shoulder the blame for policy miscalculations in the middle of the campaign which had a ruinous effect.
She offered to resign in the wake of the disaster but the political landscape had altered and her colleagues would not hear of her stepping down.
What saved the party and the leader from political oblivion was that the four PD seats, added to the 77 won by Fianna Fáil, put the two parties into a position to form a government with the support of dependable Independents.
The icing on the cake was that Charlie McCreevy, a personal friend of Harney's and an ideological soul mate on economic issues, was appointed as minister for finance. There followed the five most successful years in the history of the PDs, as the party policy of low taxation was not only implemented but accepted by all the other main parties as the correct approach.
The PDs arrived in government as the Celtic Tiger was taking off and the party benefited from the association of its own low-tax policies with the economic boom. They could argue with some justice that they helped to change the terms of political debate for good.
Ms Harney's second election in 2002 was a far cry from her first outing. Although the PDs were widely written off because two of their four TDs, Des O'Malley and Bobby Molloy, retired from politics, the party doubled its number of seats from four to eight and confounded the pundits.
Among the victorious eight was Michael McDowell who had lost his seat in the deluge of 1997 but had been brought into the government as attorney general in 1999. His election stunt of climbing a lamp-post to unveil a poster with the slogan "Single-Party Government - No Thanks" was widely credited with depriving Fianna Fáil of an overall majority and it ensured a second term for the PDs in Government.
However, the second term has not gone nearly as well as the first. Although the economy has continued to do well, the mounting criticism of public services, particularly the health service, has damaged the party, particularly because Ms Harney opted to take over as Minister for Health half way through Coalition's second term.
With Ms Harney in Health, struggling to make an impact on the intractable problems in the system, and Mr McDowell in Justice, the PDs' focus on the economy has become dimmed and the party has suffered as a result.
It has also suffered from simple voter fatigue with the Government as a whole. Although people have never been better off, the Opposition parties have managed to focus political debate on failures in public services like Health and Transport and both Government parties have lost support as a result.
Ms Harney faced the choice of leading the party into her third election in a row with the prospect of serious losses or quitting while she was still ahead.
Given the row earlier in the summer over what assurances she had or had not given that she would step down as leader before the election, there was little percentage in her staying on.
She made the point at her press conference yesterday that it would have been difficult for her to ask the electorate to support her next time out if she could not give an assurance that she intended to lead the party for the foreseeable future.
While the timing of yesterday's announcement may have come as a surprise, there can hardly have been real surprise at the decision itself. The Tánaiste has chosen the right time to go before she was trapped into reluctantly fighting another election as leader.
She made the point that she is willing to continue serving in Government and the new leader is likely to ask her to stay on in Health in the hope that she can deliver on some of her planned reforms before the election.
Whatever happens in the future Ms Harney will go down in Irish political history as a significant figure who succeeded in delivering on her party's essential programme, despite severe adversity at times.
The PDs have survived for longer than any of the other small parties that have come and gone over the years and their influence in government has been far greater.
The next election will determine whether they have the staying power to make a significant impact for at least another Dáil term.
The new leader will face great odds in bringing the party back in reasonable shape but that will be nothing new for the PDs.