Brazil's President and Workers' Party (PT) presidential candidate Dilma Rousseff gestures before a TV debate in Sao Paulo, October 19, 2014. Opposition candidate Aecio Neves is heading into the final week of Brazil's presidential race with a razor-thin lead in polls, but it's the incumbent Rousseff who appears to be gaining momentum in the homestretch. [Photo/Agencies]

RIO DE JANEIRO - President Dilma Rousseff gained some momentum but remains locked in a tight battle for votes with challenger Aecio Neves ahead of the Oct 26 runoff to Brazil's presidential election, an opinion poll showed on Monday.

Rousseff has 45.5 percent of voter support versus 44.5 percent for Neves, according to the survey by polling firm MDA, its first since the election's first round vote on Oct. 5. The difference between the two is statistically insignificant because it is within the poll's margin of error.

Excluding undecided voters, spoiled and blank survey responses, Rousseff has 50.5 percent against 49.5 percent for Rousseff.

In recent opinion polls by the Datafolha and the Ibope pollsters, Neves was slightly ahead of Rousseff, also within the surveys' margin of error.

Rousseff won the first-round vote with 41.6 percent of the votes cast to 33.6 percent for Neves, a difference of 8 million votes. Neves was endorsed a week ago by environmentalist Marina Silva, who placed third with 22 million votes.

MDA interviewed 2,002 voters over the weekend in a poll commissioned by the transport lobby CNT. The poll's margin of error is 2.2 percentage points in either direction.