Voters will feel "tricked" if DUP leader Sir Jeffrey Donaldson stays at Westminster despite seeking election to Stormont, TUV leader Jim Allister has said.
He called on Sir Jeffrey to make clear his intentions following speculation that if elected to the Assembly, he may appoint a DUP colleague to the role and retain his MP seat.
Mr Allister said: "Surely if you're standing in an election you're standing to serve - not to desert."
Read more: Stormont will never be the same again if TUV succeeds at election, Jim Allister says
The DUP is running 30 candidates for the Assembly election in May - eight fewer than in 2017.
Sir Jeffrey, who represents Lagan Valley at Westminster, is seeking election as an MLA for the constituency alongside party colleague and former First Minister Paul Givan.
If the DUP leader takes his seat in the Assembly he would have to give up his MP role, triggering a by-election.
However, with Stormont's future uncertain, there is speculation that Sir Jeffrey could temporarily co-opt someone else to the MLA post to remain at Westminster.
Mr Allister said it was clear the DUP is "not in growth mode" and "have a fear of losing some seats".
He said: "I think the key question in Lagan Valley is will Jeffrey Donaldson, whose name is on the ballot paper, will he become an MLA if elected to be an MLA for the people of Lagan Valley?
"Or will he even at that last late stage having obtained such a mandate, reject it in favour of staying in Westminster?
"I think that's a question that Jeffrey would need to answer to his constituents in Lagan Valley."
He added: "I think many voters in Lagan Valley would feel they have been tricked, which is why the question needs to be answered."
Mr Allister said TUV, which for the first time will have Assembly candidates in all 18 constituencies, is entering this election campaign in a "better position than previously going into any election".
The North Antrim MLA said unionists see TUV "as the party that got it right" in its fierce opposition to Irish Sea trade arrangements agreed by the UK and European Union under Brexit's Northern Ireland Protocol.
In an interview with Belfast Live, he said: "I think TUV has caught much of the unionist public mood, particularly on the Protocol.
"I think people see that TUV articulated it right from the beginning when some other people were talking foolishly about 'sensible' aspects within the Protocol, about 'best of both worlds', about 'opportunities'.
"TUV from day one was identifying it as a betrayal of our position as an integral part of the United Kingdom, because it annexes us into a foreign place."
Mr Allister said TUV would not take up any ministerial posts in the Stormont Executive if the party won enough seats to be entitled to join the power-sharing government.
He said his party has a "principled objection" to Northern Ireland's system of mandatory coalition, and would only ever join a voluntary coalition.
But he said TUV would never enter government with Sinn Féin, describing the republican party as "toxic and interested only in destroying not governing Northern Ireland".
He said: "I think the whole infrastructure of Stormont is absurd and I think mandatory coalition is something incapable of working, and has indeed designed the constant conflict and stand-offs that we have.
"So I'm in the business of trying to dislodge mandatory coalition and replace it with voluntary coalition and a proper opposition.
"So the taking of any office which sustains mandatory coalition is not in my vision.
"TUV would not be sustaining mandatory coalition - period."
He added: "If TUV was returned here with enough seats to hold Executive office, we wouldn't be holding it.
"We would be opting for opposition with a view to trying to thwart this process and change this process towards one which is democratically accountable."
Read more: Stormont will never be the same again if TUV succeeds at election, Jim Allister says
Read more: Crossgar Protocol rally: Unionists urged 'wake up' and maximise vote at upcoming Assembly election
For the latest politics news straight to your inbox, sign up to our newsletter here.
