BUCHAREST, Romania — Romania’s most powerful politician, Liviu Dragnea, was ordered on Monday to begin serving a three-and-a-half-year prison sentence for abuse of power, upending the political order in the East European country and handing a victory to those who view corruption as the chief threat to the health of the nation.

The ruling by the Romanian Supreme Court, which declined to revisit a sentence handed down by a lower court in June 2018, is not subject to appeal. It came one day after Mr. Dragnea’s party, the Social Democrats, were handed a stunning rebuke in elections for the European Parliament.

The Social Democrats received just 23 percent of the vote, down significantly from the 46 percent they received in the last national parliamentary elections, in December 2016. The two main opposition parties combined to secure about 50 percent of the vote.

At the same time, in a nonbinding referendum, an overwhelming majority of voters — more than 80 percent — expressed deep dissatisfaction with attempts by the governing party to change corruption-related laws via emergency decree, and potentially offering amnesty to officials convicted of abusing their power.