Analysts said Rousseff was a victim of a widespread collapse in support for the Workers' Party of her mentor, jailed former president Luiz Inácio Lula da Silva, in the south of the country.
Editor-in-Chief, Buenos Aires Times.
Brazil's leftist former president Dilma Rousseff failed to win a Senate seat in Sunday's general election, her bid sinking under a right-wing backlash.
Rousseff took only 15 percent of the vote and was running a distant fourth in the Senate race for Minas Gerais state with nearly all the votes counted, officials said.
A survey on the eve of the election had indicated that Rousseff was the favuorite to win in the southeastern state.
Analysts said Rousseff was a victim of a widespread collapse in support for the PT of her mentor, jailed former president Luiz Inácio Lula da Silva, in the south of the country.
The swing to the right was confirmed by far-right candidate Jair Bolsonaro's comfortable win in the first round of the presidential election.
Bolsonaro, who took 46 percent of the poll, said "polling problems" had prevented him from winning outright.
Instead he will face a run-off on October 28 against Fernando Haddad, from Lula's party, who took second place with 29 percent.
Rousseff – who came to power in 2010 and was re-elected in 2014 – was controversially impeached in 2016 after being accused of manipulating public accounts.