The PT was the biggest winner in the lower house, as the number of its seats increased from 79 to 88 after the election.
The PMDB lost 10 seats, a decrease from 90 to 80, but the Brazilian Socialist Party (PSB), also in the ruling coalition, had their seats increased from 27 to 36.
On the opposition side, the PSDB's presence shrank from 65 to 53 deputies, and the deputies of the Democrats Party (DEM) decreased from 54 to 43.
Although the Green Party's presidential candidate Marina Silva surprisingly garnered 20 percent of valid votes in the elections, the party's presence remained at 14 seats.
In all, the new Congress would allow Rousseff to count on a qualified majority, equivalent to three fifths of the Chamber of Deputies and the Senate, or 308 and 49 respectively, needed to approve constitutional amendments and laws.
Besides, the opposition would not reach the minimum required to create Parliamentary Committees of Inquiry (CPIs), an instrument commonly used in Brazilian politics to block the legislative process and prevent the enactment of new laws.
If elected, Rousseff "will have an unprecedented parliamentary majority to deal with her agenda," local newspaper Folha de Sao Paulo said in an article published on Tuesday.