By Luiza Ilie

BUCHAREST (Reuters) - Romania's Constitutional Court is under "shocking" political pressure ahead of its ruling on the validity of a referendum to impeach the president and has appealed to the Council of Europe for protection, a council official said on Tuesday.

Prime Minister Victor Ponta's leftist Social Liberal Union (USL) is trying to oust suspended President Traian Basescu. Ponta accuses Basescu of abusing his position to block government legislation, but the European Union regards some of the government's tactics as undermining democracy.

The row has stalled policymaking in Romania, delayed vital economic reforms and raised concerns over the Balkan state's International Monetary Fund-led aid deal.

An impeachment referendum held on July 29 yielded an 88 percent majority in favor of removing the right-wing Basescu, but the turnout fell below the 50 percent threshold required by the Constitutional Court.

The Court was expected to invalidate the plebiscite, but USL officials asserted that the real number of voters was smaller than thought, and the Court has put off its ruling to August 31 and asked to see voter lists to assess the true size of the electorate.

On Tuesday, the president of the Venice Commission - the Council of Europe's advisory body on constitutional matters - said in a statement that the court had asked his panel for help to protect its independence from political pressure.

"It is for me surprising, and even shocking, that for the second time in a short period of time the constitutional court of a member state of the Council of Europe and the European Union feels obliged to appeal to the Venice Commission in order to protect its independence," Gianni Buquicchio said.

He did not say how his commission would respond.

The letter, sent by chief Constitutional Court judge Augustin Zegrean, said that one judge had said he was afraid to vote on the referendum's validity after being threatened.

It also said the Court had been given very tight deadlines by parliament to rule on complex issues related to Basescu's suspension and the impeachment referendum.

European Commission vice-president Viviane Reding expressed support for the Court, saying in a letter to it: "Let me assure you that the European Commission is following the situation very closely and is committed to ensuring respect for the rule of law and the independence of the judiciary in Romania."

Brussels has various ways of putting pressure on Romania, monitoring its judicial system, and keeping it outside the passport-free Schengen zone for now. Romania gets EU cash to help it catch up with other members and the bloc contributes to its IMF-led deal.

EU, U.S. PRESSURE

Romania's foreign ministry said late on Tuesday that threats to judges would not be tolerated and urged judges to report any such incidents to the authorities.

"Romanian government authorities ... will not make any kind of exception from strictly respecting the independence of the judiciary," it said in a statement.

Before the July 29 vote, Ponta's government tried to reduce the high court's powers and ignore the 50 percent turnout rule, and threatened to replace judges who were not compliant, until concerted pressure from the European Commission, the United States and Germany forced it to back down.

The updated electoral list may show the number of registered voters was smaller than thought and thus push turnout above 50 percent, but most analysts doubt this will happen.

Basescu has lost popularity because of his support for the austerity policies required to restore confidence under the IMF aid package, and over the view that he has engaged in cronyism.

Ponta called on Basescu to resign after the referendum, citing the 88 percent vote against him. The president replied that a majority of voters did not want him removed from office and he has refused to step down.

The dispute has pushed the leu currency to record lows and, the IMF says, could further damage the recession-ridden economy.

"We are a little bit worried about the situation and also the impact the political situation has on the economy," IMF mission chief Erik de Vrijer told interim President Crin Antonescu, a Ponta ally.

(Reporting by Luiza Ilie; Editing by Tim Pearce)