By Tom Perry

BEIRUT, Aug 22 (Reuters) - Bitter rivalry between Lebanon's political camps and between their respective foreign backers could torpedo next month's presidential election, threatening a new constitutional crisis, instability and economic paralysis.

The poll is the next battle in a struggle that pits the ruling coalition backed by the United States and Saudi Arabia against a Shi'ite-Christian opposition including the powerful Hezbollah group, which enjoys Syrian and Iranian support.

According to the constitution, parliament should meet on Sept. 25 to elect a successor to President Emile Lahoud, a close Syrian ally whose term ends at midnight on Nov. 23.

But the vote is unlikely to happen then or before the end of Lahoud's term without a settlement of the political standoff which has paralysed Lebanon since November and triggered outbreaks of strife reminiscent of the 1975-1990 civil war.

"Everybody is awaiting the positions of the Americans and the influential regional states," a senior Lebanese politician said. "All the talk on the local level is just to pass time."

Syria's insistence on extending Lahoud's term in 2004 raised tension in Lebanon and provoked U.S.-led pressure on Damascus.

Replacing Lahoud with someone independent of Syrian tutelage has been a priority for supporters of Prime Minister Fouad Siniora's government since Syrian troops left Lebanon in 2005, amid an outcry over the killing of ex-premier Rafik al-Hariri.

Hezbollah is equally determined to stop the presidency falling into the hands of political adversaries it says are controlled by Washington.

REGIONAL CONFLICTS

Breaking the Lebanese impasse thus depends on easing wider tensions -- a slim prospect right now, given U.S. sparring with Syria and Iran, recent animosity between Syria and Saudi Arabia, and Sunni fears of Tehran's rising influence in Iraq and beyond.

"There needs to be a regional thaw -- between Syria, Saudi and Iran and Saudi," said Lebanese political analyst Oussama Safa said. "I doubt there's a chance of that soon."

The pro-government majority bloc made up of Sunni, Christian and Druze factions accuses the opposition of seeking to restore Syrian dominance in Lebanon. Hezbollah sees Siniora's cabinet as a tool of U.S. policy in the Middle East.

Opposition leaders are demanding a national unity government in which they would hold veto power before any vote on a new president, who they say must be a consensus choice.

"No national unity government, no presidential elections," leading Hezbollah cleric Sayyed Hashem Safieddin said this week.

Druze leader Walid Jumblatt, a fiery anti-Syrian politician, rejected any such government. "I am against a compromise," he told Reuters. "It means political suicide for us."

Yet neither side can impose its own candidate.

The pro-government bloc cannot ensure a two-thirds quorum in parliament. The opposition would reject any president elected without a quorum as unconstitutional.

The president must be a Maronite Christian in Lebanon's power-sharing system, but the Maronite community is divided over the declared candidates, who include opposition leader Michel Aoun and pro-government figures Boutros Harb and Nassib Lahoud.

The name of army chief Michel Suleiman has been floated as a compromise candidate, although the constitution would need to be amended to let him assume the post straightaway. Central Bank Governor Riad Salameh is also seen as a possible neutral choice.

STEP INTO UNKNOWN?

Lahoud, like the opposition, disputes the legitimacy of the Siniora government and says he will not turn over power to it if his term expires before a new president has been elected.

Some analysts say Lahoud may appoint a new government in competition with Siniora's -- or even try to stay in office.

Jumblatt said it was too early to predict the outcome, but suggested that if the deadlock persisted the ruling coalition could hold a vote outside parliament without opposition MPs.

A senior opposition source said such a step would "take Lebanon to the unknown" and would be confronted "by all possible means". Even some pro-government MPs have questioned whether such an unprecedented election would have any legitimacy.

Amal Saad Ghorayeb, an analyst with the Carnegie Endowment's Middle East Center, saw no prospect of polls taking place in the fraught atmosphere clouding Lebanon.

"Once Lahoud's term expires, he could hand power over to the army commander," she said.

That would mirror a move in 1988, when former President Amin Gemayel named then army chief Aoun to head a military government in conflict with the existing cabinet. Syria drove Aoun from office in 1990, with a green light from Washington.

"The creation of two governments will recreate the dangers of civil war and they (the opposition) are not ready to go with this alternative," prominent Lebanese columnist Sateh Noureddin said, pinning his hopes on foreign pressure to secure a deal.

"We are only at the beginning of this political circus." (Additional reporting by Laila Bassam)

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