President Klaus Iohannis blasted the measure put forward this week by the justice minister, saying the ruling Social Democrats "don't care about Romania; they care about their own politicians with legal problems."
Premier Viorica Dancila is considering the measure, which could lead to the suspension of trials and verdicts and lead to resentencing in the cases of some convictions.
In televised comments late Tuesday, Dancila said she supported the measure, insisting it wouldn't impede the anti-corruption fight.
But critics say it would be a setback for democracy and weaken efforts to fight high-level graft.
The development came after the Constitutional Court ruled in November that one of five judges on the High Court for Cassation, the nation's highest appeals court, wasn't appointed properly in 2014, opening the way for it to review verdicts issued since 2014.
"All these people who have suffered deserve a fair trial," Dancila said. "It's sad that people went to prison. Their families and health were destroyed."
Iohannis said "laws can't work retroactively" in a country where there is rule of law.
An EU official said Brussels was watching the development closely.
"It is essential for Romania to get back on track in the fight against corruption to ensure an independent judiciary and to avoid any further steps back," Margaritis Schinas, EU Commission spokesman said Tuesday.
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