RIO DE JANEIRO—Brazilian President Michel Temer signaled Thursday he may scale back a proposed overhaul to the social security system, as opposition from legislators threatens to derail the linchpin of his plan to revive the economy.

The pension reform, which seeks to establish a minimum retirement age of 65 years and reduce generous payouts and perks, would fail to pass Congress if a vote were held now, a survey of legislators by newspaper Estadão found on Wednesday. Only 94 of 513 legislators in the lower House support Mr....