Sinn Fein is more concerned with protecting its own votes than the vulnerable, Alliance Party leader David Ford has said.
He accused republicans of turning 360 degrees over welfare reform.
Mr Ford addressed his party’s annual conference this afternoon in Co Down.
“Whoever is being more disingenuous, cynical or stupid, let me be absolutely clear where the blame will lie if this agreement falls apart over welfare reform.
“It will lie with every MLA who signed last week’s petition of concern - Sinn Fein, SDLP and Green Party.
“Don’t let them get away with their claims that it’s all about protecting the vulnerable, because it’s not. It’s about protecting their own votes.”
On Monday Sinn Fein withdrew support for welfare reform legislation as it was about to go through the Assembly.
Martin McGuinness claimed it would target the most vulnerable and hurt future claimants.
Measures to mitigate cuts in benefits were a key element of the Stormont House Agreement which was signed before Christmas by the five political parties and the British and Irish governments.
Now proposals to devolve corporation tax powers to Stormont are threatened.
Mr Ford said: “This week’s crisis raises even bigger questions about the future of the Agreement, even of the institutions.
“I don’t know who to be more frustrated with. Is it the SDLP, who sat silent throughout the Stormont House discussions on welfare reform, didn’t object to the agreement that was presented to the Government as the basis for all the other financial commitments, and then pursued a shamelessly populist strategy as the Welfare Bill made its way through the Assembly?
“Or is it Sinn Fein, who have now turned a full 360 degrees, opposing welfare reform, agreeing to implement it, and now opposing it once again?”
He said politicians could not protect vulnerable people if they had to take another £200 million or more out of public services to fund further adjustments to benefits.
“We can’t protect the vulnerable if we have to take hundreds of millions out to pay fines to the Treasury. And we can’t make the step change in job creation if we can’t afford to reduce corporation tax.
“Their actions will damage jobs, damage public services, and damage the most vulnerable. Not so much left-wing as incompetent.”