Helen Clark is trying to persuade Labour supporters against strategic voting as an increasing number of polls show the Green Party is making inroads into Labour's support base.
When she was asked at a meeting with Foodstuffs workers if they should vote strategically to ensure a Labour coalition, she said the best way was to vote for Labour.
She said Labour's vote was close to where it was in 1999.
"If we can turn out the Labour vote, we will be leading the next Government. This is a down-to-the wire election and we have a very real chance of doing it again. We need to mobilise our heartland."
Helen Clark has been working to shore up the blue-collar vote - speaking to warehouse staff at Progressive Enterprises in Auckland and Foodstuffs in Rotorua.
The unions have also galvanised into action, waiting for her to appear at public events and handing out pamphlets alongside her. The National Distribution Union's leaflet for members will go out next week - advising them to vote for Labour, the Greens or Maori Party.
The Green Party announced it would consider siding with Labour in post-election negotiations and recent polls have shown Labour is losing votes to it.
Yesterday National leader John Key continued his campaign, saying any coalition Labour could cobble together would be a "five-headed monster" which would make difficult the decisions needed to handle the economy.
Speaking to reporters in Greymouth yesterday, Mr Key said voters should be wary of a coalition government that could include Labour, the Progressives, the Greens, New Zealand First and the Maori Party.
"Do [New Zealanders] want to put in a National government with a fresh view that will work going in one direction with a small group of parties, or do they want a potentially five-headed monster?"
His comments came in the wake of a TV3 poll, which last night showed a Labour-led coalition with support from the Green Party, the Maori Party and the Progressive Party could govern.
In return, Helen Clark said Mr Key was "getting a bit rattled" and fought back with her own predictions about what a National/ Act government would mean.
"He knows any arrangement from National that brings Roger Douglas anywhere near government is something people do not want at all. So he's trying to cover that very unpopular fact."
She said Mr Key's decision to rule out Winston Peters was the sign of a money trader's short-term thinking.
She said he was also trying to "stitch up" a government with four parties - Act, United Future and the Maori Party.
"I think it's a bit a pot looking for a kettle to call black."