Epsom is a lifeline to Act, because Prime Minister John Key has asked National voters to give their electorate vote to Act candidate David Seymour.
Mr Key has ruled out a deal with the Conservatives, meaning it would have to gain at least 5 per cent of the party vote to enter Parliament - a feat that Act does not have to achieve.
Mrs Rankin came out swinging for a National-led Government, which could be seen as a pitch to David Seymour voters.
"National needs the Conservatives to give them a backbone, and keep them on a short leash," she said.
She denied she was trying to win votes off Mr Seymour, saying she was "not interested in Act".
Mrs Rankin said internal polling showed the electorate was socially conservative, with drug and alcohol policy being the biggest concern. She wanted the drinking age returned to 20, and a total ban on so-called legal highs. "The lowering of the drinking age has been very bad. It hasn't worked. I've certainly got no wishy-washy views about drugs. We do not agree with liberalisation.
"They [the Act Party] most certainly are liberal and we are not."
Mrs Rankin lives on the North Shore and considered standing in Upper Harbour against National's Paula Bennett, but the party needed a presence there weeks ago and she was fighting pneumonia.
"Now I have a clean bill of health. I don't think it's a prerequisite to live in the electorate. The Prime Minister doesn't live in his electorate. The Leader of the Opposition doesn't, either."