Jens Stoltenberg's shock return to Norway politics upends polls before election – Arbeiderpartiet surges ten points ahead of September vote

The polling surge really is quite fascinating. Labour hasn't seen polling like this since 2018.

Stoltenberg's return is definitely part of it. He's more popular and respected than Støre among voters, and a bit of a media darling, so his comeback has given Labour a lot of free publicity. The article mentions another factor:

His return coincided with the exit from the government in late January of the eurosceptic Centre Party, which had limited Labour's margin of manoeuvre. Bergh said that had contributed to the party's surge in the polls.

The Centre Party did really well in the previous election (though they were on a steady downward trend in the polls even before that), and has used their position to force through a bunch of more or less stupid reversals of the previous government's reforms. This made Labour look both weak and incompetent, causing voters to flee. (The Centre Party is also up in the polls since the split, but less so than Labour.)

A third factor, not mentioned in the article, is that immediately after the break-up, Labour announced a new extremely populist pricing scheme for electricity, which has exploded in price in parts of the country since the outbreak of the Russia-Ukraine war. Electricity prices has been the most important issue for many voters since 2022.