A decision by Labour's Hutt South MP Trevor Mallard to stand on the list only next election makes it more likely the seat will fall to National.
Mallard has had a strong personal following in the seat, having held it since 1993.
But in the 2014 election, National's young candidate Chris Bishop reduced Mallard's majority to just 709, from 4825 in 2011.
Mallard would also have picked up votes from local Green candidate and list MP at the time Holly Walker, who was retiring from politics.
The greater indication of where the seat could head in next year's election in in the party vote: National overwhelmingly won the party vote with 17,648, well ahead of Labour on 10,903.
Even with the Greens added to Labour, National was ahead on the party vote by more than 2000.
Mallard, who is currently assistant Speaker, says his decision is about wanting to become Speaker in the next Parliament and he had decided it was very hard to chair the House in an unbiased way and be an effective electorate MP.
Labour leader Andrew Little has said that Mallard would be Labour's nomination for Speaker if it forms the next Government.
Prime Minister John Key said Bishop had been extremely active in Hutt South and that "Trevor Mallard is waving the white flag".
Mallard's decision does not make it any more difficult for Labour to form a Government, however.
Even if Bishop were to win Hutt South, the total number of seats each party has in the Parliament is decided by the nationwide party vote, not electorate seats.