The leader of the DUP has said he would not support a government which is captive to a separatist party after the General Election.
Peter Robinson was asked would his MPs join forces with the SNP at Westminster.
The DUP was the largest party from Northern Ireland represented in the Commons with eight seats.
Mr Robinson said: "We would not support any administration that was going to be captive to a separatist party, we are unionist, we want to retain the UK.
"If a government was being propped up by a separatist party that was using its position in order to extract levers of separation then clearly we could not support that.
"However, if the ask of any party that forms such a support group for a government party is for purely regional issues then that is a very different matter indeed."
The DUP has issued a five-point plan it will use as part of any negotiations following a hung parliament.
Scrapping what opponents of welfare reform call the bedroom tax is a key demand.
Mr Robinson has refused to say how much money for Northern Ireland the party would be asking for.
He said his MPs were likely to exercise influence during a tight race.
"As close as they are at the present time, it is very hard to see that we would not have some role in the future.
"It is not just if a party needs to get a certain number to top them up to get over that line.
"This is a five-year parliament, they need to have sufficient to be able to run through the five years so even if there was a party that had sufficient numbers to form a government on its own it will not be by such numbers that they could ignore parties such as ourselves."
The DUP has signed a pact with the Ulster Unionist Party to run joint candidates in North and East Belfast, Newry and Armagh and Fermanagh and South Tyrone aimed at keeping out rival parties.
It will face a challenge from smaller unionist parties. The hardline Traditional Unionist Voice (TUV) will contest seven seats while Ukip will also field candidates.
Mr Robinson has ruled out being part of a formal coalition but has claimed the DUP is the best placed of Northern Ireland's parties to exert influence in a less formalised arrangement if the outcome is as tight as some pollsters predict.
Not only is it the most well-represented in the Commons, but Sinn Fein - the second largest Westminster party with five seats - does not take up its voting rights and has insisted it has no intention of reversing its century-old abstentionist policy come May, despite some speculative media reports suggesting a deal with Labour had been mooted.
The SDLP won three seats in 2010, but with its traditional alignment with the Labour Party it would not have the DUP's flexibility to negotiate with David Cameron and Ed Miliband.