"Somebody has thrown something burning through a letter box," said Mikael Ericsson at Sörmlands police.
The fire was extinguished immediately and by the time the emergency services arrived only smoke remained. Police have classified the case as arson or attempted arson.
Frida Grundström, who is a member of the municipal council in Gnesta, was home alone in her apartment when the incident occurred at around 8pm.
"I stood in the kitchen making pizza and had some music on. But then I heard the fire alarm and went out into the hall to see what had happened. When I came out there it was burning on the door and on the floor," Grundström told the Dagens Nyheter (DN) daily.
The apartment has been cordoned off and will be examined by police forensics.
According to the newspaper Grundström has previously been subjected to harassment and assault, and has had a swastika daubed on her front door.
The politician is convinced that the attack had a political motive.
"It is not going to feel safe and secure to return to the apartment when the police have completed their work," Grundström told DN.