Act and United Future would support the sale of up to 49 per cent of five state-owned enterprises, the $5 billion to $7 billion from which is central to National's economic plan.
The Greens and the Maori Party oppose the asset sales, although the Maori Party says it would also support iwi that wanted to buy a share to keep the assets in New Zealand ownership.
National has largely refrained from criticising its three support partners.
And until now it has been fairly gentle on the Greens.
But yesterday National turned the blowtorch on them when campaign chairman Steven Joyce said their policies would drive New Zealand further into debt by billions at the worst possible time.
Nine of the Greens' policies would add $25 billion to debt over four years in addition to Labour's debt, he said.
Mr Key said that it was important to understand what was on offer.
"On our side it's stable parties we've worked with who deliver stronger economic growth and won't load the country up with debt.
"The four political parties that could form a government on the left of us all have big spending promises or would put a lot more debt on the NZ economy and would threaten jobs."
Green co-leader Russel Norman said he was surprised to receive a lesson on fiscal responsibility from the National Government - "a Government that recently received a double credit downgrade in response to their management of the New Zealand economy".
He said the policies Mr Joyce had costed had not been priority areas the Greens would be seeking to change after the election.
"With numbers like those, Steven Joyce should consider an alternative career with Lehman Brothers."
