According to a survey by TNS Gallup for Norway's TV2 network, the centre-right's lead has shrunk to just 12 seats in the 169-seat parliament, down from 48 before the summer break.
The pollster expects the two smallest right-wing coalition partners, the Christian Democratic Party and the Liberal Party, to fall below the four percent threshold to enter parliament.
A rival survey published on Tuesday by the VG newspaper, however, put the centre-right on track for a 43-seat lead, which it said would be a "disaster" for the ruling Labour party.
With just over a month to go before the September vote, the polls are a crucial gauge of the scale of the task faced by Prime Minister Jens Stoltenberg if he wants to stay in power for a third term.
The TV2 figures, he said, were "a source of inspiration and show that nothing has been decided."
As for the VG figures, he said: "I have seen both good and bad measurements, and I do not intend to let myself be influenced by the bad ones."