The Prime Minister says National minister Murray McCully would stand aside in Auckland's East Coast Bay's electorate if he asked him to.
John Key has yet to decide whether National will do a deal with the Conservative Party to help them into Parliament.
Conservatives leader Colin Craig is running in East Coast Bay, a seat which has been held by Mr McCully since 1987.
Mr Craig has said before that he expects National to stand aside for him.
Mr Key told reporters on Friday that the decision would be made with select Cabinet ministers, including Mr McCully.
"We obviously believe National can govern the country far more effectively than the very far left coalition of parties. So in the end, I think Murray would do what was in the best interests of the party.
"But he's making the obvious point which is he's been the very long standing member of East Coast Bays - and a very effective one."
Mr Key said he would spell out any deals to voters well and truly before the election on 20 September.