BUENOS AIRES — The recovery of a corpse this week in a river in Patagonia has shaken up Argentina in the final stretch of a high-stakes midterm election, amid widespread speculation that it is the body of Santiago Maldonado, an indigenous rights activist missing for more than two months.

The remains were found on Tuesday less than 1,000 feet upriver from where Mr. Maldonado, 28, was reported last seen on Aug. 1 during an indigenous rights protest that was broken up by security forces.

Mr. Maldonado’s ID was found on the body, his brother, Sergio Maldonado, said at a news conference Wednesday night, although relatives were awaiting the results of a forensic examination to confirm the identity.

“Until I am 100 percent certain I will not confirm it,” Mr. Maldonado said hours before the body was flown to Buenos Aires for an autopsy, which was scheduled to begin Friday morning.