For 13 years the Workers' Party has shaped Brazil under presidents Luiz Inacio Lula da Silva and Dilma Rousseff. However, Rousseff's removal from the presidency in an impeachment vote this August and Lula's battle against corruption charges have accelerated a stunning decline in fortunes.
Blamed by many for Brazil's deep recession, the party is expected to lose City Hall in the biggest city Sao Paulo as well as a swath of other posts in the 5,568 municipalities holding polls.
Sunday's polls are the first electoral battle since Rousseff was replaced by center-right rival Michel Temer in the presidency and will be the last rehearsal before 2018 general elections.
A spate of political killings is adding to tensions lingering from the impeachment crisis. The latest victim was a mayoral candidate shot dead while campaigning Wednesday in Itumbiara in the central state of Goias.
Worries about violence have prompted deployment during the election of extra army and elite police forces to 266 municipalities in 11 states, according to Globo news site.