By Tom Perry - Analysis

BEIRUT (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 September 25 to elect a successor to President Emile Lahoud, a close Syrian ally whose term ends at midnight on November 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 paralyzed 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.