DUP leader Gavin Robinson has invited the incoming UUP leader Jon Burrows for talks on unionist co-operation.
The invitation comes a day after the former senior police officer emerged as the sole contender to succeed Mike Nesbitt as UUP leader.
In some previous elections, unionist parties have agreed to step aside in specific constituencies in a bid to maximise the unionist vote.
Sinn Féin currently has the highest number of MLAs in the Assembly, entitling the party to nominate a first minister, while the DUP nominated a deputy first minister.
The party also secured the most number of votes of any of the Northern Ireland parties at the 2024 Westminster election, securing seven seats, ahead of the DUP's five.
In a letter to DUP party members, Mr Robinson referred to what he called Sinn Féin's "fervent desire to bring about the removal of Northern Ireland from the UK".
"As leader of the largest unionist party, I am determined to make every effort to ensure that unionism does not squander the narrow window of opportunity that exists to prepare for the future," he said.
"I support, and want to reflect the desire across the grassroots of unionism, to see unionist representatives and their political parties working together to promote the union, to stand up for all those who want to get on with their lives and who want to see our country prosper," he added.
Mr Robinson said unionism's collective voice and clout "will be stronger" if it works together, and cautioned against a divided movement.
"The truth is that if unionism has more seats, it will have more power to get things done," he said.
"The opposite is also true, a divided and fractious unionism without co-operation will cost seats and our collective influence will diminish. I stand ready to act in the widest interests of the pro-union cause and to work across the unionist spectrum.
"I have written to the incoming leader of the Ulster Unionist Party setting out our position and inviting him to meet," Mr Robinson said.
"I look forward to working with Jon Burrows to see how our parties could co-operate and work towards building greater support for the union and to test how we might best strengthen pro-union co-operation leading to the election of more pro-union members at local government, Assembly and Westminster elections," he added.
Mr Robinson said he had met previously with TUV leader Jim Allister, and that he knows "we stand ready to co-operate to maximise unionist representation".
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"It is the job of all of us to be building support for the union and making the case for it. We must do that at home as well as nationally and internationally.
"The truth is that Sinn Féin is not interested in working with the UK government or the rest of us who at our core want to do what is best for Northern Ireland," he added.
"They find every excuse to be themselves alone, to duck difficult decisions, and to follow an all-Ireland anti-United Kingdom agenda," Mr Robinson said.
"Whilst Sinn Féin have little to show for their unity campaign, that does not take away from their fervent desire to bring about the removal of Northern Ireland from the UK and our ultimate destruction as one of the four nations bound together in the Union.
"By working co-operatively, unionism can impact and thwart their objectives whilst bringing about real benefit to the everyday realities of everyone in Northern Ireland," he added.
Earlier this month, Mr Burrows spoke of his desire to grow the UUP, and said while he was not opposed to unionist co-operation, he was not in favour of being part of any unionist merger.
"I'm a unionist who is strong in unionism. I've proven that. But I'm here to work with everyone to deliver for the people of Northern Ireland," he said.
"I want the Ulster Unionist Party to once again be the biggest party in Northern Ireland. That's my ambition. And the people of Northern Ireland need an Ulster Unionist Party, with reasonableness and decency and integrity and that is rooted in finding solutions to the everyday problems that Stormont has failed to deliver on," he added.
