Mr Davis said if Labour was polling at about 30 per cent "then I'm on the cusp, and Hone can't use the line, 'Vote for Hone and you get two MPs'. It'll be head to head."
He said it was a democracy and the Maori Party could stand if it wished. "Good on him [Mr Shortland] but I won't wish him luck. If the Maori Party wanted to get rid of Hone, then they were silly to stand somebody."
Maori Party president Pem Bird said the selection of Mr Shortland - a Ngati Hine leader, broadcaster and actor - would restrict Mr Harawira's ability to campaign nationally for the Mana Party rather than stay at home to defend his seat.
Mr Bird said the selection of Mr Shortland had him believing the Maori Party could win the seat back, despite his earlier doubts.
"After getting a bit of a spanking, it would make you doubt that to be truthful. But Waihoroi will give a very good account of himself. So if Hone believes he will be freed up to go round the countryside and promote Mana, I'd have another think about that if I was in his shoes."
Mr Shortland was the only person nominated for the Maori Party candidacy. All Maori Party candidates are expected to help pay for their campaigns and show they have supporters to form a campaign team.
Mr Harawira and Mr Shortland did not respond to a request for comment.
Mr Shortland will stand down as chairman of his runanga to campaign and will resign if he wins the seat. Ngati Hine is the largest Ngapuhi hapu and Mr Shortland is spearheading a challenge against the iwi's settlement board, Tuhoronuku, for the right to negotiate Treaty claim and fisheries settlements.
The Maori Party and Mr Harawira originally had an agreement not to stand against each other in the Maori seats.
However, talks broke down and the Maori Party decided it would contest Te Tai Tokerau. Mana is yet to announce its full candidate list but John Minto, Sue Bradford and Annette Sykes have all confirmed they will stand.