The Act Party has already assured National it will support it on confidence and supply after the next election.
Leader Don Brash said last night that he had given that assurance to Prime Minister John Key last month, five months before the election, in the interests of stability.
Although Act had many policy differences with National, "clearly it is inconceivable that we would want any other party to lead the government".
Former Act leader Rodney Hide made the same pledge to National before the last election but much closer to polling day.
Dr Brash said giving the pledge did not imply Act would support every policy a National Government would put up - just the Budget and confidence motions - and Act would still enter post-election talks.