EU referendum: Rival parties join forces to step up the Remain campaign
They're partners in opposition at Stormont, now the SDLP and UUP have united to campaign for a Remain vote in the EU Referendum.
The South Belfast MP Dr Alasdair McDonnell joined newly-elected South Antrim MLA Steve Aiken and veteran MEP Jim Nicholson at a business breakfast to discuss the case for the UK staying in the European Union.
Dr McDonnell said that EU membership had opened up significant trading opportunities for companies in Great Britain and Northern Ireland and he was worried about the impact of a Brexit on the economy.
"The notion that Britain could strike new trade deals with European countries shortly after a Brexit vote is just insane," said the former SDLP leader.
"Negotiating these arrangements is a highly skilled job and I'm not convinced that we even have these skills in Whitehall.
"From a Northern Ireland perspective, I don't want to see checkpoints on the border or at our ports and airports and that's what would happen. If you don't have a hard border north-south, there will have to be one at the points of entry and that's equally disturbing.
"It will be bad for the sort of private sector growth that we're working hard to encourage in Northern Ireland. Two thirds of our exports go to EU countries and we must ensure that we have a channel to Europe without any barriers."
Dr McDonnell said he was deeply concerned about how a Brexit might affect the peace process in Northern Ireland.
"Our safety and security would be threatened," he said. "The 1998 Good Friday Agreement brought us stability and I don't want to see violence returning. Dissidents of various hues and from both sides could try to take advantage of that."
The UUP's Steve Aiken, a former Royal Navy submarine commander, said that the dual EU membership of the UK and the Republic of Ireland had underpinned the peace process.
"A Brexit would change that and our mutual interests would diverge because the Republic will not be voting to leave the EU, despite what some people on the Leave side are claiming," he said.
"I believe a Brexit would trigger another Scottish referendum, and for us in Northern Ireland that would be nothing short of a catastrophe. It would have an impact on our sense of identity and would cause huge uncertainty. We need confidence in the future."
Mr Aiken, who's also a former chief executive of the British-Irish Chamber of Commerce, told the 80 guests at the breakfast in the Malone Lodge Hotel that a Brexit would create more problems for business than it would solve.
"Leaving the EU will not free us from trade regulations. We would still have to meet all of the EU's rules if we still want to export to the EU," he said.
"Those on the Leave side keep talking about opening up trade with nations outside of the EU, but our ability to crack China and India is limited and it will take until the middle of this century at least to build up a significant level of trade with those countries.
"If we vote for Brexit, we will manufacture our own recession."
Jim Nicholson, who's been an MEP for 27 years, said he believed that a Brexit would be a "disaster" and that instead, Britain should adopt a new attitude towards the EU.
"I want to see the UK leading in Europe. We must end this half-in, half-out approach," he said.