Jonas Andersson, Chairman of the Sweden Democrats in Linköping, says he was restrained by two people before a third punched him in the face.
"I had been out at the pub and drank two beers. After I said goodbye to my friend, two guys flew towards me", he told Swedish television.
He said he believed the attack was politically motivated because one of the men had asked him to confirm that he was a member of the Sweden Democrats, before threatening him, saying "there is no tomorrow for you".
The attack follows recent criticism of the nationalist party after another local election candidate quit, when photos emerged of her wearing a Nazi swastika armband.
On Saturday a parliamentary candidate was involved in a racist video row.
It is understood that police have questioned a man on suspicion of the assault in Linköping. He is believed to have denied the allegations against him.
Led by Jimmie Åkesson, the Sweden Democrats are currently on track to win more than ten percent of the vote in Sunday's elections.