A cynical change of heart pours scorn on their claims to be party of government
Ready for government in the Republic - that was the boast of Mary Lou McDonald and Gerry Adams at the Sinn Fein party conference at the weekend.
But, as Sinn Fein reneges on agreements it has made on welfare reform and puts the entire future of the Assembly in doubt, they look less and less like a party of government.
There will be negotiations, but Stormont could fall and an election won't cure this mess.
The alternative to agreement will be some form of direct rule from Westminster.
That would be a shocking abdication of responsibility by well-paid ministers and MLAs and it would be ironic if Sinn Fein ended up pushing us closer to Britain with no local Assembly.
That isn't the intention of most republicans but they are drifting into dangerous territory. Already, they and the DUP are accusing each other of bad faith, and relations which Mr McGuinness recently said were better than ever have now hit rock bottom again.
Mr McGuinness's task at the ard fheis was to convince members and voters watching on the TV that, after years of practising with the DUP in the north, Sinn Fein were now well fit to run the country, either in partnership or on their own.
Mr McGuinness laid it on with a trowel to delegates. He went through all the figures, figures which show that everyone won't be automatically protected from changes in welfare spending forever. He went on: "The categories of welfare recipients we have protected include children with disabilities, adults with severe disabilities and families who would have been affected by the benefit cap. In addition, the punitive bedroom tax has been neutralised. No-one in the north of Ireland will pay a bedroom tax.
"We will not be part of an agenda which pushes more children into poverty or targets the disabled as a way to save money. That is why we negotiated and agreed to protect those benefits under the control of the Assembly, so that the categories of claimants targeted by the Tories in Britain will be protected by the unique measures we are putting in place here."
Yesterday, it was a different story. "The DUP have acted in bad faith and are now reneging on their commitments to protect the most vulnerable. It is their intention to provide only partial protection to current recipients of benefit and no protection whatsoever for future claimants. That is totally unacceptable," he said.
That sounded as credible as some unionists did suggesting they had never heard of the On the Run Scheme. If Sinn Fein wants to assess the impact scuttling this agreement has on their partners, they should think of the day the DUP unexpectedly vetoed the Maze Peace and Reconciliation Centre.
Both moves created distrust but, economically, yesterday's decision on welfare reform is far more threatening. As the Secretary of State has pointed out, it endangers our next Budget as well as our economic plans and the low interest loans from London. These are loans we were forced to take on when the Executive spent money it couldn't raise and ran up hundreds of millions in debt.
Sinn Fein seem to have missed the point that all we got for welfare reform was the right to pay claimants out of our own resources, something which could cripple us if we don't keep a handle on it. It involves paying people here under an older and, currently, more generous benefit system. However, it would be rash to guarantee to keep up this legacy system forever - there could be future changes in benefits for one thing.
Before he made his speech at the ard fheis on Friday night, the Deputy First Minister had been in Stormont waiting for figures on the cost of extending the safety net. Sinn Fein was given them around 4pm, according to the DUP, when Mr McGuinness returned to his ard fheis to say how well things were going. He says he also expected to meet the DUP, but the point was that he had the figures and, anyway, the Stormont Agreement was clear enough, even specifying a review.
If he knew things were as bad as he is now making out, why did he tell his party members that we had achieved a better welfare system than Britain? It is not the sort of behaviour that creates trust, either amongst voters or potential partners in government.