A letterbox smear campaign against Shellharbour Labor candidate Anna Watson threatens to derail the party's bid in its only safe Illawarra seat.Anonymous leaflets have been distributed to a number of homes around Warilla, Oak Flats, Shellharbour and Dapto over the past few days.The leaflet features a photograph of Ms Watson and a number of accusations which attempt to cast aspersions on her character and fitness for office.A spokeswoman for Ms Watson said she was "shocked and upset by this unauthorised, illegal and offensive material being distributed in her local community"."[These] sorts of dirty tactics don't belong in this campaign," the spokeswoman said.She said the campaign team was "considering potential legal action [against] the distributors of this material" but said the identity of the perpetrators was not known.Asked whether Ms Watson would respond to whether the claims were true, the spokeswoman said "I don't anticipate that".Matters of unauthorised electoral material would also be referred to the NSW Electoral Commission, she said.A Mercury-IRIS poll published on Saturday found Ms Watson, a unionist, was the only Illawarra Labor candidate who would comfortably win their seat at the state election in three weeks' time.It predicted she would take the seat with an 11 per cent margin - a 15.8 per cent swing against the ALP.Ms Watson replaces outgoing incumbent MP Lylea McMahon, who announced her resignation last year for family reasons.However, it is widely believed Sussex St forced Ms McMahon to step down after intense pressure from the union movement, which had opposed a head office decision to install her before the last election.Ms Watson defeated nurse John Rumble, son of former state MP Terry Rumble, in a close preselection contest last December.