By Ned Parker
BAGHDAD, July 1 (Reuters) - Newly elected Iraqi lawmakers convene on Tuesday, under pressure to name a unity government to keep the country from splitting apart after an onslaught by Sunni Islamists who have declared a "caliphate" to rule over all the world's Muslims.
The meeting of the new legislature in Baghdad's fortified "green zone" could spell the end of the eight-year rule of Shi'ite Islamist Prime Minister Nuri al-Maliki, with foes determined to unseat him and even some allies saying he may need to be replaced by a less polarising figure.
Iraqi troops have been battling for three weeks against fighters led by the group formerly known as the Islamic State in Iraq and the Levant (ISIL). Fighting has raged in recent days in former dictator Saddam Hussein's home city, Tikrit.
ISIL, which rules swathes of territory in an arc from Aleppo in Syria to near the western edge of Baghdad in Iraq, has renamed itself simply the Islamic State. It declared its leader, secretive guerrilla fighter Abu Bakr al-Baghdadi, to be the "caliph", the historic title of the ruler of the whole Muslim world.
Its insurgency in Iraq is backed by other Sunni armed groups who resent what they see as persecution under Maliki.
The new parliament is meeting for the first time since it was elected in April, when results initially suggested it would easily confirm Maliki in power for a third term.
But with lawmakers now finally taking their seats only after the sudden collapse of the army in the north, politicians face a more fundamental task of staving off the collapse of the state, and the prime minister's days in power could be numbered.
Maliki's foes blame him for the rapid advance of the Sunni insurgents who seized the biggest northern city, Mosul, on June 10 and have since taken nearly all the Sunni areas of the country.
Although Maliki's State of Law coalition won the most seats, it still needs allies to govern. Sunnis and Kurds demand he go, arguing he reneged on power-sharing deals and favoured his own sect, inflaming the resentment that fuels the insurgency.
Washington has not publicly called for Maliki to leave power but has demanded a more inclusive government in Baghdad as the price for more aggressive help.
In another move to beef up its military presence in Iraq, the United States said on Monday it was sending 300 more troops to Iraq.
U.S. Defense Department spokesman Rear Admiral John Kirby said about 200 forces arrived in the country on Sunday to reinforce security at the U.S. Embassy, its support facilities and Baghdad International Airport. A further 100 personnel were also due to move to Baghdad to "provide security and logistics support."
"These forces are separate and apart from the up to 300 personnel the president authorized to establish two joint operations centers and conduct an assessment of how the U.S. can provide additional support to Iraq's security forces," Kirby said in a statement.
FIGHTING IN TIKRIT
Maliki's government, with the help of Shi'ite sectarian militias, has managed to stop the militants short of the capital but has been unable to take back cities its forces abandoned.
The army attempted last week to take back Tikrit but could not recapture the city, 160 km (100 miles) north of Baghdad, where ISIL fighters machine-gunned scores of soldiers in shallow graves after capturing it on June 12. Residents said fighting raged on the city's southern outskirts on Monday.
Whether Iraq can survive as a state most likely depends on whether politicians can sustain the governing system put in place after the U.S. invasion that toppled Saddam in 2003, under which the prime minister has always been a Shi'ite, the largely symbolic president a Kurd and the speaker of parliament a Sunni.
On Friday, in an unusual political intervention, Grand Ayatollah Ali al-Sistani, Iraq's most senior Shi'ite cleric and a figure long known for his caution, called on political blocs to fill those three posts before parliament meets on Tuesday.
That deadline looks futile, with the blocs having met in recent days without naming the country's leaders.
Two senior members of Maliki's State of Law coalition have told Reuters that an alternative to Maliki from within his party was being discussed: "He understands it might come to that," one senior Maliki ally told Reuters last week. Maliki's own former chief of staff Tareq Najem is seen as a possible successor, according to diplomats.
The Sunni parties say they will not put forward their candidate for speaker until they see who the Shi'ites want for prime minister. The Kurds have yet to choose a president.
"It will take a couple of weeks to agree on a package," said Muhannad Hussam, a politician and aide to senior Sunni lawmaker Saleh al-Mutlaq.
Many worry that a drawn-out process will waste precious time in confronting the militants, who have vowed to advance on Baghdad. A Shi'ite lawmaker, speaking on condition of anonymity, warned: "Things are bad. The political process is not commensurate with the speed of military developments." (Additional reporting by Phil Stewart and Missy Ryan in Washington; Editing by Peter Graff and Peter Cooney)