Hadi Mizban, Associated Press
July 15, 2014 - Shiite lawmaker and Deputy Parliament Speaker Haider al-Ibadi speaks to the media after an Iraqi parliament session in Baghdad. On Monday, Aug. 11, 2014, Iraq's largest coalition of Shiite political parties chose al-Ibadi to be its candidate to lead the government in a major defeat for incumbent Prime Minister Nouri al-Maliki just hours after he declared himself the rightful candidate and put troops on the street.

BAGHDAD — Under heavy pressure from the United States, Iraqi lawmakers took a significant step on Monday by choosing a replacement for Prime Minister Nouri al-Maliki, widely blamed for their country’s polarized politics. But al-Maliki angrily rejected the move, vowing to fight in the courts and perhaps by use of force, throwing the country into new uncertainty even as it fights an onslaught by Sunni militants.

The change in leadership could help soothe Iraq’s sectarian fractures and unite the country under al-Maliki’s nominated successor, a member of his own Shiite party. But al-Maliki’s insistence that he is the rightful leader could just as easily tear Iraq further apart.

Further complicating the picture was the United States, which helped orchestrate al-Maliki’s rise to power eight years ago but now holds him responsible for alienating the country’s Sunni minority and helping fuel the rise of the Islamic State, the Sunni extremist group. Territorial gains by the Islamic State in the north prompted a new military intervention by the United States — and gave the U.S. fresh leverage to demand political changes in Baghdad.

President Barack Obama welcomed the nomination of a new prime minister, Haider al-Abadi, interrupting his vacation on Martha’s Vineyard to announce in a televised statement that both he and Vice President Joe Biden had congratulated al-Abadi on the phone, calling his nomination “an important step towards forming a new government that can unite Iraq’s different communities.”

But Obama also reminded the Iraqis that America’s renewed military assistance — punctuated by the airstrikes that began pounding Islamic State positions last week — was no solution to what he called the larger crisis in Iraq. “The only lasting solution is for Iraqis to come together and form an inclusive government,” he said.

Although al-Maliki is widely reviled in Iraq, he remains a formidable force, with relatives who command special security forces, courts that are heavily shaped by his influence and a history of exacting revenge on his domestic opponents. Al-Maliki’s stubbornness presents multiple challenges to the United States, which wants to preserve Iraq’s cohesion while helping to stop the Islamic State’s avowed goal of creating a monolithic Islamic caliphate that ignores national boundaries.

Obama spoke after a day of high political drama in Baghdad, where al-Maliki appeared on state television and blamed the U.S. for “standing beside those who violated the Iraqi Constitution.”

“We will fix the mistake,” he said, without being specific.