A few years after the multiparty system ended, and he chose the MDB (Brazilian Democratic Movement) and in this party was elected state representative in 1966 and mayor of Campinas in 1968.

In 1974, he was elected senator after a tied competition with Carvalho Pinto that was appointed as favorite running for re-election with the ARENA (National Renewal Alliance Party).

In 1982 he was elected vice governor of São Paulo, but differently from the image of party unity shown during the campaign, was constant against PMDB politicians linked to the governor, he did not succeed but tried to prevent the Congressman Mário Covas to be the mayor of São Paulo in 1983 and the election of Senator Fernando Henrique Cardoso to the presidency of the state directory of the PMDB that same year.

It was adherent to the Diretas Já and the winning campaign of Tancredo Neves toward the presidency in 1985, the year he married the doctor Alaíde Cristina Barbosa Ulson.

After the victory of former President Jânio Quadros (Brazilian Labour Party-PTB) over Fernando Henrique Cardoso in November of that year, Orestes Quércia saw his popularity in the party increase with the movement called "quercismo" which guaranteed its nomination as candidate for governor despite internal dissidence.

His government was responsible for a big growth economical[1] of the state and for privatization of VASP in 1990, the year in which elected Luiz Antonio Fleury Filho[2] as his successor.

Quercia was accused of stealing material from the Roads Department (DER) to build fences on his farm in Pedregulho-SP, of importing electronic equipment without competitive bidding of Israel, overbillings works of the subway, and irregularities in the privatization of VASP.

Quércia placed 4th in the 1994 presidential election with 2,773,793 votes (4.4%), behind the victorious Fernando Henrique Cardoso of the PSDB (54.3%), Luiz Inácio Lula da Silva of the PT (27%), and Enéas Carneiro of the PRONA (7.4%).

Quercia invested in properties and in the communication business – was the owner of Sol Investment Group,[5] which owns and control Brazil FM Radio Nova, the newspaper Financial DCI, two regional broadcasters, TVB Campinas (from February, an affiliate of Rede Record) and TVB Santos (from March, affiliate Rede Bandeirantes), the Shopping Jaragua and several farms.