According to the best surveys(SOM questionnaire) SD was the most trusted party in immigration issues, and their stance of lessened immigration had much more support than the alliance+green immigration policy. If you make that the biggest issue of the election then SD will gain.

That was the plan. Benefit SD on the cost of the left wing to prevent the left wing majority that the polls were showing. Worked too.

Also positioning yourself as the anti-SD is a vote-gainer as the Feminist Initiative showed in 2014s EU election. SD is the most disliked party by a large margin.

And the current liberal migration policy also sizeable support, though in proportion to parliament votes SD has most to gain from an election centering on immigration.

chumchu:

By not agreeing to be a support party SD became the only opposition party

I don't think to much should be read into that. They have claimed to be one party against the rest for years, so while the DÖ played into their rethoric I think the effect was limited.

chumchu:

they are now polling 25-30%.

Not quite.

Poll of polls from Svensk Opinion:

Bad enough as it is.