When Mikael Jansson defected from the Center Party and took over Sweden Democrats in 1995 he began a purge of the openly extreme-right elements of the party in order to turn it into a respectable anti-immigration movement, but it didn't really solidify until the past few years. Even in recent years they have had a struggle to keep those types out of the party, for example their youth leader from 2007-2010 (Erik Almqvist) was embarrassingly videotaped singing white-power music at a retreat, and was finally forced out of the party earlier this year (weirdly, for someone who opposes immigration, he reportedly emigrated to Hungary after resigning).
DF by contrast has different origins, culturally rooted in something more like old-person Danish culture, rather than youth radical-right culture. They have a vaguely mythologized idea of 19th-century or early-20th-century Danish culture, as they imagine one's grandfather in a small town in Jutland might have lived. They generally therefore view the radical right-wing scene as negative, as equally a deviation from traditional quiet Danish values as immigrant crime is. This has let them keep those elements out of the party more credibly, because it's not only for image that they do it, but because their base actually culturally dislikes the far-right (especially anyone associated with the metal or biker scenes, who they see as culturally un-Danish).
Not that I myself would vote for DF (as an immigrant in Denmark they aren't out to make life easier for me), but I somewhat understand their appeal better; they are more of a rural "traditionalist values" party than a radical nationalist party. I find it harder to understand how SD would get similar support, unless people either don't realize their association with the far-right subculture, really believe that their transformation is completed, or are just lodging protest votes.
Because in Denmark, immigration can be part of public debate without being shunned, while in Sweden, it's hard to get any sort of ground in this debate without being labelled a racist.
So now, some of them are trying to be more 'reasonable' about it with the party Sverigedemokraterna. I assume because they haven't made any progress publicly in Sweden by just being seen as extremists. And more and more figures are beginning to tell Swedes that immigration is not an issue you can ignore, thus Sverigedemokraterna's support is rising, because it's the only party that wants to talk about it.
But I am confident, that most of its votes are 'protest votes', to get the other parties to talk about it.
