By GORDON FAIRCLOUGH
BUCHAREST—Romania's Constitutional Court ruled that a referendum aimed at ousting the country's president was invalid, clearing the way for his return and setting the stage for further political conflict in this young Eastern European democracy.
The tribunal's finding in favor of President Traian Basescu also marked a victory for the European Union, which had criticized efforts to remove the unpopular center-right politician as a serious threat to the rule of law here.
In a 6-3 decision, the court on Tuesday said that turnout for the July 29 recall was below the 50% threshold required for it to be legally binding. Election officials said 46% of voters cast ballots; Mr. Basescu's opponents questioned the accuracy of the voter lists.
Parliament—which suspended Mr. Basescu in early July, saying he had overstepped constitutional bounds—must now formally approve his reinstatement. Analysts expect that to happen soon.
Mr. Basescu, a 60-year-old former ship captain, has described the effort to eject him from office as an attempted coup by the left-leaning governing coalition led by Prime Minister Victor Ponta, a 39-year-old Social Democrat.
On Tuesday, Mr. Ponta said he would respect the ruling, but said the referendum results showed Mr. Basescu was "an illegitimate president." He said he would propose constitutional amendments to improve "checks and balances" on presidential power.
More than 85% of those who participated in the referendum voted against Mr. Basescu, whose standing with the public has suffered because of his support for painful steps to cut the budget deficit and allegations of cronyism.
The EU said Tuesday it would closely monitor adherence to the court decision and called on "all political forces to respect European values, to act with responsibility and to work constructively in overcoming divisions."
Intense partisan feuding has consumed Romanian politics for months, sparking fears among investors that needed economic measures won't move ahead and pushing the Romanian currency, the leu, to record lows against the euro this summer.
"Continued tension is the most likely outcome" of Mr. Basescu's return, said Otilia Simkova, a London-based analyst for political risk consultancy Eurasia Group. Ms. Simkova predicted a heated campaign ahead of parliamentary elections, due by November.
The battle between Mr. Ponta's left-leaning government, which took power in May after the fall of two previous center-right premiers, and the president escalated in June, after a major victory in local elections by Mr. Ponta's camp.
A series of maneuvers by Mr. Ponta and his allies—including emergency decrees and other steps that would make it easier to get rid of Mr. Basescu, drew fire from the EU and U.S., which said they threatened democratic checks and balances.
Under pressure from the EU, Mr. Ponta agreed, against the wishes of some in his coalition, to enact as law a requirement that a majority of registered voters participate in the recall referendum in order for it to be valid.
That change proved critical for the outcome. Mr. Basescu urged his supporters to stay away from the poll. That, combined with the large number of Romanians living and working abroad, made the turnout standard very difficult to meet.
"Sometimes the EU needs a very big stick" to prevent democratic backsliding among members, said Sorin Ionita, a political analyst at Bucharest-based public-policy think tank Expert Forum.
Mr. Ionita said he believes that at the heart of the efforts to unseat Mr. Basescu is a desire by politicians to thwart changes aimed at cracking down on corruption. Mr. Basescu has backed antigraft prosecutions.
Former leftist Prime Minister Adrian Nastase was recently sentenced to prison after being convicted on corruption charges.
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