By Radu-Sorin Marinas

BUCHAREST (Reuters) - Romania's anti-corruption prosecutors said on Monday they had begun a criminal investigation of two prominent Social Democrat party lawmakers suspected of collaborating to produce a bill on amnesty and pardons in criminal cases that would benefit one of them.

The investigation of Senator Dan Sova and Viorel Hrebenciuc, the vice president of the lower house, by the Directia Nationala Anticoruptie (DNA), emerged two weeks before a Nov. 2 presidential election. Prime Minister Victor Ponta is widely expected to win and consolidate the position of the Social Democrats as the ruling party.

On Oct. 17, Hrebenciuc used his influence to prompt Sova to propose a bill on amnesty and pardoning of sentences, the DNA said in a statement. If passed into law, it could have have protected Hrebenciuc from prison in another case.

"In exchange, Hrebenciuc promised Sova he would persuade some members of his party in his 'spheres of influence' to back him (to get the party's head post)," the prosecutors said.

Neither Hrebenciuc nor Sova were immediately available when Reuters placed calls to their mobile phones. Local television Realitatea TV showed Sova saying he "would not comment for the moment" and Hrebenciuc saying "I just took note" of the investigation.

Influence peddling is a criminal offence under Romanian law. It can carry penalties of up to seven years in prison.

The prosecutors were already investigating another case involving Hrebenciuc, related to recovering forest land seized under the communist dictatorship. Last week, prosecutors widened that investigation to include Ponta's father-in-law, Ilie Sarbu.

These officials were accused of supporting an organised crime group in a case where roughly 43,000 hectares (430 sq km) of forest land was illegally granted to third parties in 2012.

Thousands of Romanians are still waiting for compensation or the return of property seized under communism before Stalinist leader Nicolae Ceausescu's violent overthrow in 1989. Disputes over land ownership, inefficiency and red tape have hampered efforts to return land to their rightful owners.

On Monday, prosecutors said the two lawmakers, Sova and Hrebenciuc, had allegedly "discussed in detail" accusations brought against Hrebenciuc in the forest land investigation, and about "the risk of (Hrebenciuc) being convicted."

There have been no suggestions that Ponta - a former prosecutor himself - is involved in any of the cases. He told reporters: "Everyone is equal before the law."

Analysts have said such allegations are unlikely to harm Ponta's performance in the upcoming elections. The latest opinion surveys give him a strong lead over his top challenger in the first round and a winning majority in the Nov. 16 runoff.

The European Union has raised concerns about a failure to tackle rampant high-level graft in Romania and Bulgaria, its two poorest members. Both have been kept outside the passport-free Schengen Zone since entering the EU in 2007.

Romania ranks behind only Greece and Bulgaria in perceptions of corruption in the 28-nation EU, according to Transparency International. The European Commission has its justice system under special monitoring.

Ponta has weathered several political storms since coming to power. Shortly after he became prime minister in 2012, he faced down calls to resign over accusations he had plagiarized his doctoral thesis. He said the charges were politically motivated.

(Editing by Larry King)