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.
Dr Brash also said Act and National had confirmed the same agreement in Epsom as they had under Mr Hide - that National would campaign primarily for the party vote.
He said Act would perhaps be stronger if its candidate won Epsom with no assistance from National, but it was better to give voters a sure way to elect a stable Government "even if it slightly weakens Act's ability to drive a hard bargain in post-election talks".
"We have a duty to ensure stability."
Former Auckland city mayor and National cabinet minister John Banks is standing for Act in Epsom, replacing Mr Hide.
Dr Brash, the Reserve Bank Governor for 14 years, also criticised the Tea Party Republicans in the United States, saying they had "played chicken with the world's largest economy".
While he agreed that spending had to be brought under control, "in our judgment the Tea Party people were so one-eyed that they were not willing to discuss a constructive way out of what in the US is a very serious fiscal dilemma".
"The economy needs confidence. In the last few weeks, businesses all over America have put away their cheque books as events in Washington have made people scared to invest, to hire and to buy.
"The damage to the American economy from the debt debacle may turn out to cost the US Government more in lost revenue than the spending savings the Republicans achieved."
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Labour list MP David Parker has been confirmed as Labour's candidate for the Epsom electorate.
Mr Parker has been a Dunedin list MP.
He will face Mr Banks, a former National cabinet minister and Auckland City mayor.By Audrey Young Email Audrey