BEIRUT - Deepening political splits in Lebanon are expected to thwart an attempt to choose a new president on Wednesday, with the powerful Shi'ite group Hezbollah mobilising against a bid by the main Christian parties to elect a senior IMF official.

It is the 12th time parliament has convened to fill the presidency, a post reserved for a Maronite Christian in Lebanon's sectarian system that has been vacant since Hezbollah-allied Michel Aoun's term ended in October.

Amid intensifying sectarian tensions, the main Shi'ite factions are poised to block the move by groups including Lebanon's two biggest Christian parties to elect Jihad Azour, a former finance minister and the IMF's Middle East director.

Lebanon's government vacuum is already unprecedented. The country has neither a head of state nor a fully empowered cabinet that might address a crippling financial meltdown.

Hezbollah and its closest allies are expected to withdraw from the session to deny a quorum for the vote.

The group, which says it is exercising its constitutional rights, is backing its close Christian ally, Suleiman Frangieh, a friend of Syrian President Bashar al-Assad who strongly supports Hezbollah's right to possess weapons.

Hezbollah, which is designated as a terrorist group by the United States, has unleashed fierce rhetoric in their campaign against Azour, describing him as a candidate of confrontation.

Lebanon's Shi'ite Mufti Sheikh Ahmad Qabalan dialled up the attacks on Sunday against Azour without naming him, accusing him of being backed by Israel and saying "a president with an American stamp will not be allowed".

Azour, 57, has said he wants to build national unity and implement reforms in a country mired in its deepest crisis since its 1975-90 civil war.

Azour served as finance minister from 2005 to 2008, a period of political conflict pitting a government backed by the West and Saudi Arabia against opponents aligned with Damascus and led by Hezbollah.

His backers include the anti-Hezbollah Christian Lebanese Forces party, and the Free Patriotic Movement led by Gebran Bassil - a Hezbollah ally with presidential ambitions whose ties to the group have been strained by its backing for Frangieh.

The sectarian split was on show on Tuesday evening when Bassil said nobody could bypass Christians in the choice of president, and warned of the risk of the "country and the noble Shi'ite sect" being drawn into a religious and national conflict, though he said this would not be allowed.

Bassil said his party was in a growing dispute with Hezbollah, which he urged "to stop any intimidating and threatening language" and halt efforts to peel FPM lawmakers away from voting for Azour.

Azour also has the backing of the Progressive Socialist Party led by the Jumblatt family, and some Sunni Muslim lawmakers.

Hezbollah's sway in parliament - where 128 seats are divided equally between Christian and Muslim groups - suffered a blow last year when the group and its allies lost a majority.

Parliament needs 86 lawmakers to meet quorum and a first round of voting also requires 86 votes to win. In the following rounds, only a simple majority - 65 votes - is needed.

(Reporting by Maya Gebeily; Editing by Tom Perry and John Stonestreet)