Ulster Unionists want a radical change to the power-sharing formula in Northern Ireland.
The alteration to the Good Friday agreement would see them form a voluntary coalition government with the nationalist SDLP.
But speaking at his party's manifesto launch leader David Trimble said he could not consider going into government with Sinn Fein.
He said republicans had shown they were not prepared to wind down the IRA.
The UUP's proposed changes are at odds with the SDLP's stated position.
The party ruled out government without republicans at its manifesto launch earlier on Wednesday.
"It is clear, moreover, that the unionist electorate would not support, or tolerate now, or in the foreseeable future, the formation of an executive that would include Sinn Fein.
"We have called for the d'Hondt formula that automatically gives parties ministerial appointments to be repealed.
"In that event, we would seek to form a voluntary coalition with the SDLP.
"That would give voters the chance to form a cross community administration leaving out the extremists, if they want it and vote for it."
The UUP's suggested change would amount to a radical rewriting of the way the Good Friday Agreement states a government can be formed in Northern Ireland.
The main points of the UUP manifesto include:
Campaigning for the retention of the full-time police reserve and abolishing the 50:50 rule for recruiting equal numbers of Catholic and Protestant officers
Insisting on the use and strengthening of anti-social behaviour orders
Supporting a statutory victims' charter to enhance and protect the rights of those who suffered during the Troubles, ensuring their needs were paramount
Backing a United Kingdom-wide identity card scheme in the fight against terrorism and crime, to reduce welfare and electoral fraud and prevent illegal immigration
Supporting an anti-racism strategy for Northern Ireland to promote British values of tolerance and inclusion.
Demanding the same investment as other parts of the UK in water and sewerage services
Opposing the government's plan for water charges based on capital value
Calling for a fairer rates system, ensuring any new system for calculating rates protects the most vulnerable from becoming asset rich but cash poor as house prices rise
Replacing the province's four health boards with one authority, developing community hospitals to offer a wide range of services
Advocating a ban on smoking in all public places
Substituting the five education boards with one authority, freeing up money spent on red tape for the classroom
Guaranteeing a free pre-school place and promote a fairer method of academic selection making the most of children's potential
Create a unified farm inspectorate to replace existing schemes.
Developing a new partnership between Government and the fishing community in the face of EU reforms;
Pressing for a new flat rate pension of £110 a week, and replacing means tested benefits