Ian Paisley says strong unionism is more important than ever after Taoiseach Enda Kenny's shock announcement that he would consider a coalition with Sinn Fein.
he Fine Gael leader stunned his own colleagues with his dramatic shift in position on Gerry Adams's party, less than a year after saying it "wasn't fit for government".
But this week Mr Kenny reacted to a change in Sinn Fein's stance about its must-have position as the major player in any coalition. The shift was signalled by deputy leader Mary Lou McDonald when she said she wants to be in government and a "conversation" needs to be had between now and the next election.
Mr Kenny, who has been leader of Fine Gael for 15 years, has always maintained there was no way his party would work with Sinn Fein. But asked whether this might change in light of Ms McDonald's comments, he replied: "I said I wouldn't do business with Fianna Fail so, depending on the result you gave as a member of the electorate, politicians have to work with the result. So Sinn Fein seem to be converted now to a position of changing their stance."
He later said Sinn Fein have "a long journey to go".
A number of senior Fine Gael sources were shocked by Mr Kenny's inconsistency, with one saying: "This is ridiculous stuff. I can't understand why he would open up this can of worms."
And politicians in Northern Ireland reacted strongly too, with Mr Paisley saying the only answer was for unionists to vote for the DUP.
"The thirst for power and holding on to power is always unquenchable, so I think ultimately if Sinn Fein is the largest party in the South of Ireland then Fine Gael will end up doing a deal with them," said the DUP MP.
"This is clearly Sinn Fein policy, to be the All-Ireland party. And if that's the policy, then we need to make sure that unionism is held up as being the largest party in Northern Ireland.
"Unionists must calculate that the only way to ensure that our ideology and strategy and policies prevail is by supporting the only unionist party that can lead Northern Ireland."
Mr Paisley also said the change in position from Fine Gael shows a "public abandonment" of any link with the SDLP.
"The SDLP leader must feel absolutely devastated this morning in the wake of comments that signal Enda Kenny is willing to speak to Sinn Fein, with a view to doing a deal with Sinn Fein," he added.
Ulster Unionist leader Mike Nesbitt said: "For many years we have had a totally unacceptable position from the Irish Government that was 'do as I say, not as I do'.
"When we brought forward the Belfast Agreement in 1998, we promoted democracy and exclusively peaceful means over wholesale slaughter on our streets. In doing so, we realised the difficult challenge of finding ways of working with parties of different philosophies.
"What we did not contemplate was attempting to gain party political advantage from power-sharing, it would be disappointing if that was what was motivating this potential change in policy."
Sinn Fein said: "The Sinn Fein position in relation to entering coalition government is a matter for the party's ard fheis. This issue is a topic of ongoing debate among the membership, from the leadership to the grassroots. This is healthy for a growing party such as ours and it is a conversation that the leadership is encouraging. It is a conversation that we need to have. However, any decision to change the current position can only be taken by the membership at an ard fheis."
The SDLP declined to comment.