Riot police use a water cannon during protests in Bucharest on Aug. 10. (Vadim Ghirda/AP)

THE SLOW erosion of democracy in Central Europe advanced another step last week, when Romania’s corruption-ridden government resorted to brute force to answer protests against its subversion of the rule of law. After tens of thousands of people gathered in central Bucharest to call for the government’s resignation, riot police launched an assault with a water cannon, tear gas and batons, beating peaceful participants, journalists and even several Israeli tourists . More than 400 people were reported injured — a shocking toll for a political protest in a member state of the European Union and NATO.

The employment of violence was just the latest tactic of the ruling Social Democratic Party (PSD) in what has been a relentless campaign to shield itself from consequences for rampant graft and other criminal activity. The biggest beneficiary is its leader, Liviu Dragnea, who already has two criminal convictions on his record, including one in June for putting PSD hacks on the government payroll.

That result came about thanks to the strenuous efforts of Laura Codruta Kovesi, the chief of an anti-corruption directorate that Romania created five years ago under heavy E.U. pressure. In all, Ms. Kovesi and her staff won the convictions of more than 1,000 officials , including nine former cabinet ministers and scores of parliamentary deputies. Her reward was to be forced from office last month by Mr. Dragnea’s crew, which also passed laws decriminalizing low-level graft and weakening the independence of prosecutors and judges.

The encouraging news here is that average Romanians, like people in Poland opposed to a similar governmental power grab, have been pushing back. Last year, a massive demonstration by hundreds of thousands forced the government to back down from an initial attempt to change graft laws, and there were more demonstrations at the end of last year . Last week’s protest was driven by the large Romanian diaspora, whose remittances from work in more prosperous parts of Europe are vital to the economy. Despite the government’s violent response, demonstrators returned to the streets on several following nights — and the riot police were shamed into withdrawing.

The PSD’s assault on the rule of law could still be turned back by Romania’s constitutional court. Not surprisingly, the popularity of Mr. Dragnea and his party is declining, but the next parliamentary election could be more than two years away. That means the cause of Romanian democracy needs more help from the E.U. leadership and that of other Western democracies, including the United States. The European Commission should step up pressure on the government to nullify or drastically amend the new laws. And the Trump administration should sanction Romanian officials involved in corruption and the use of violence against peaceful demonstrators under the Magnitsky Act, which was designed to address such abuses. A historic achievement, the consolidation of democracy in former communist states of the Soviet bloc, is in jeopardy in Romania, Poland and Hungary. The Western nations that helped foster it must do more than watch.