Countrywide, nearly 139 million Brazilians are registered to elect 5,561 mayors and 48,000 municipal councilors among 450,000 candidates representing more than 20 political parties.
While the Workers Party is not favored to win most major cities, the poll is expected to boost President Dilma Rousseff, who took office from party founder and mentor Luiz Inacio Lula da Silva in early 2011.
Casting a shadow over the elections is a trial involving a congressional vote-buying scheme that spawned the state's biggest corruption scandal in 20 years.
Thirty-seven former ministers, lawmakers, businessmen and bankers are facing prosecution before the Supreme Court over the alleged scheme that ran from 2002 to 2005 during Lula's first term.
While Lula was cleared, the scandal nearly cost the 66-year-old his re-election in 2006.
The trial, which is receiving daily television coverage since it started two months ago, "will not have a big impact" on balloting, said Alberto Almeida, a public opinion expert and president of the Sao Paulo consulting firm Analise.
It also has not hurt Rousseff's popularity, who, together with her government, enjoys high approval ratings.
Still, Andre Cesar, a political scientist at the Brasilia-based Prospectiva consulting firm, said the trial could affect the vote in Sao Paulo, home of the main Workers Party defendants -- including Lula's former chief of staff, Jose Dirceu, who was found guilty by four Supreme Court justices of running the vote-buying scheme.
Analysts will closely watch what happens in Sao Paulo, where despite the active support of Lula and Rousseff, Workers Party candidate Fernando Haddad trails in third place.
He lags behind frontrunner Celso Russomanno, a populist former television consumer advocate backed by a powerful evangelical church, and Brazilian Social Democratic Party (PSDB) standard bearer Jose Serra, an ex-presidential candidate.
Cesar described those backing Russomanno as conservative members of the new lower middle class, lacking much education and without political, labor or social ties.
Many Sao Paulo voters also see Russomanno, nicknamed "the consumers' sheriff" as a breath of fresh air and an alternative to the old Workers Party-PSDB dichotomy, he said.
In Rio, meanwhile, incumbent Eduardo Paes, a Workers Party-backed member of the centrist PMDB party, is favored to win re-election and thus remain at the city's helm during the 2016 summer Olympics.
The elections will show "a strengthening of President Dilma and her allies, as well as of her governing style, while Lula who left office in 2010 as an icon will lose a bit of his power if he does not manage to get (his protege) Haddad into the second round in Sao Paulo," Cesar said.