Alliance leader David Ford has accused the SDLP of propping up Sinn Féin, and portrayed the DUP as a party that supports the Belfast Agreement. He also said there was little or no difference between the Ulster Unionist Party and the DUP.
Mr Ford, launching his party's election manifesto in Belfast yesterday, said Alliance was the only centrist party battling for seats.
He did not predict that Alliance would win any of the 18 Westminster seats, but said he expected to enhance the party's position in the local elections.
While Alliance supporters voting tactically could play a key role in assisting Ulster Unionists such as Lady Sylvia Hermon and David Burnside fighting marginal seats against the DUP, Mr Ford said there was no UUP candidate who occupied the political middle ground.
In any case, the DUP was now effectively pro-agreement, he argued, and it was possible there could be political progress after the election, even if the Rev Ian Paisley's party was the dominant unionist force.
"Having watched the DUP since the last Assembly election come round to a position of support for the principles of the agreement in all but name, I do not think that that is necessarily a major blow to the prospects of people moving forward.
"I think the real difficulty has come when two unionist parties have competed for the extremes; the way Ulster Unionists have attempted to take over DUP ground in recent years.
"But I believe there is a clear sign that the DUP have become a lot more realistic, and therefore there is still the prospect of real movement regardless of which unionist party comes out on top," added Mr Ford.
As with the UUP and DUP, he urged the SDLP to join in a voluntary coalition in the Northern Executive that implicitly excluded Sinn Féin.
"It seems to me that while the SDLP engages in rhetoric against Sinn Féin it has actually, in practical terms, being propping up the indefensible position of Sinn Féin in allowing there could be no movement unless Sinn Féin was part of that movement," said Mr Ford.
"What it would take for Sinn Féin to take part in any executive government would be to demonstrate clearly that they have the same understanding of peace and democracy as other parties have."
There was not much purpose in the SDLP being "an advocate for Sinn Féin", said Mr Ford.
"The only way we will make progress is when those who can have confidence in each other work together, and I believe the SDLP are part of that."
He said "all tribal politicians" were a danger to society, and only by building the centre ground could society be made better.
"The polite, tribal politicians are just the same kind of danger as are the impolite, tribal politicians."
Mr Ford said he expected that Alliance would improve on its last local election result when it won 28 council seats, and that it would continue to hold the balance of power on Belfast City Council.
The key proposals in the Alliance manifesto are:
• voluntary coalition
• reducing Assembly from 108 to 80 members
• greater support for integrated education
• tougher enforcement of laws against "gangsters, rioters and vandals"
• greater North-South co-operation for practical purposes
• affirmative action to replace 50:50 Catholic/Protestant police recruitment
• abolish university tuition fees
• investment in transport.