United Future deputy leader Judy Turner says the National Party is regarded as "cold-hearted, tight-fisted and downright mean" in its treatment of the less well-off.
The party's leader Peter Dunne has declared he will support National after the election, not Labour in whose Government he is the Revenue Minister.
Ms Turner made the comments when releasing the party's social policy.
"I think even National MPs will admit that historically there have been times in history where they have taken a very hard line on some of New Zealand's citizens that were least able to cope with that harshness," she told Radio New Zealand.
"And I think there'll be people within National that are very glad that they've got an option of United Future rather than just ACT."
National Party leader John Key brushed off Judy Turner's comments, saying they are based on a historical view, and won't affect the two parties' relationship.
"Politics has always had its fair share of colourful expressions and language, I'm sure our relationship runs deeper than some colourful language from Judy Turner."