The seat is crucial to internet-Mana, which needs Mr Harawira to win or face oblivion unless the party can pass the 5 per cent threshold.
And the race is close; a Maori Television/Reid Research poll this week saw Mr Harawira and Mr Davis only 1 percentage point apart.
Mr Peters said Mr Harawira had sold out to Kim Dotcom.
"I think that's enormously destructive of not just New Zealand politics and the democratic environment, but it's very destructive of Maoridom and what it stands for.
"I think he's almost defeated the reason for having the Maori seats in the first place by his behaviour."
Mr Davis, who was also campaigning in Paihia today, said he was trying for every vote.
"I'm happy for an endorsement from anybody.
"I need 29 to 30 per cent of the [party vote to get in on the list], and I'm the one person in Parliament who knows he can't rely on the party list, having missed out last time. I'm going out to win the seat."
Mr Peters, who has not stated a preference for Labour or National in any post-election deal, said his endorsement was not a sign of cosying up to the Labour Party
"You should not read anything besides my admiration for someone who's got what it takes to be a fine representative for the north."
Earlier in Kaikohe, former Labour minister Dover Samuels showed his support for Mr Peters by joining him as he visited local supermarkets.
Mr Samuels told reporters he thinks Mr Peters should go with National after the election.
"I think New Zealanders would rather have a government that's stable. That's a reason why a number of people are supporting Winston, because they will see him as an influence of change in bringing back a National Government to middle New Zealand."
But he said the decision was up to Mr Peters.
Mr Samuels also took issue with Mr Harawira's arrangement with Kim Dotcom.
"We've got an MP who's Mana-munching and all he wants to do is have a marriage with Dotcom, a guy who shouldn't even be in the country and is using our Maori electorate as a lever."
He did not think Labour would do well on Saturday.
"You don't have to be a prophet to bloody well figure out how they're gonna vote. Labour will probably get 25, 26 [per cent]."