Arlene Foster must keep Sinn Fein onside while keeping DUP united
As I write it seems a racing certainty that Arlene Foster will be appointed leader of the DUP this evening, the first female unionist leader from Northern Ireland or Ireland.
It is quite an achievement, but she had to change parties to do it and she will need a sure hand to keep control of the DUP at a time of political crisis.
Ms Foster started off in the UUP, working as a solicitor, and there was bad feeling when she left for the DUP six years ago along with Jeffrey Donaldson.
It tilted the balance of power in the DUP's favour and the party is now more popular than ever, but facing an election within months. That makes it an even more challenging moment to take charge.
It is easier to fall from the peak of 38 seats the DUP scored last time than it is to improve on it. Besides, her former colleagues in the UUP have now moved into Opposition - there is no talk of unionist unity these days and they are likely to make it a grudge match in the Assembly.
From the UUP's point of view, she sold it out; from hers, she was being held back by a group of men in grey suits bent on monopolising positions of influence and appeasing terrorism.
The Good Friday Agreement, with its provisions for the release of prisoners and power-sharing with Sinn Fein, was the formal reason for leaving.
Ms Foster may well have the skillset to handle these problems. She is a tough customer, in some ways a female equivalent of Peter Robinson, who can be charming, cutting or dismissive as the moment demands.
She doesn't have quite the same cool, strategic brain as her predecessor, but she is getting there, and he is 67 while she is 45 and in her prime. She can count on his strong backing - he appointed her to deputise for him twice - and, if she needs it, he will be someone to consult.
In fact, no significant figure has opposed her. The original plan had been to run her for the post of First Minister with Nigel Dodds, MP for North Belfast, being the overall leader. Instead he has stayed on to be her deputy leader, and others who thought of standing then thought better of it.
She has also mended her bridges with the UUP, at least to some extent. In the most recent Westminster election both parties supported the UUP's Tom Elliott as candidate and she canvassed for him. The result was that he took the seat from Michelle Gildernew of Sinn Fein.
The biggest task of all will be to keep Sinn Fein onside while keeping her own party united.
Barring an electoral landslide, Sinn Fein and the DUP are the only two parties capable of giving Northern Ireland a stable, cross-community administration. Falling out in a major way makes them both losers if it spells the end of Stormont.
Trust fell when Robinson promised Sinn Fein a peace and reconciliation centre at the Maze, but pulled out when he found the DUP divided. Ms Foster needs to be careful not to promise what she can't deliver, and to always deliver what she does promise.
She would be a very remarkable person if she did not have mixed feelings when dealing with republicans. She was born Arlene Kelly in July 1970 and, as a child, had first-hand experience of the violence that engulfed Northern Ireland during the Troubles.
The IRA tried to kill her father, a part-time RUC reservist and farmer, when she was only a schoolgirl. She later told the Sunday Tribune how he was shot closing in the cattle. "He came crawling into the house, blood streaming down his face," she said. Her father survived, but the family had to move house.
She had another close brush when the IRA attacked the school bus she was travelling in as a teenager and saw friends injured. The bus had been targeted because the driver was a part-time UDR member.
Those sorts of experiences are hard to set aside, so providing a stable Government with Sinn Fein will test her.