As his already limited political support crumbled, the incumbent prime minister, Nouri al-Maliki, dug in for a fight. He argued that the appointment of the 62-year-old Abadi as prime minister-designate was legally invalid.
The clash has raised deep concerns at home and abroad about Iraq's teetering stability. Ominously, Maliki reminded the country in a televised address Monday of his position as head of the armed forces and assured soldiers that the "error" will be rectified.
For a second day, blue-and-white armored personnel carriers belonging to security forces that answer directly to Maliki were stationed around the Green Zone. A hulking tank sat at one of the entrances to the secured zone, which houses government buildings. The armed forces remained on high alert, officials said, though Saad Maan, a spokesman for the Baghdad Operations Command, maintained that the deployment was routine.
"We are entering a potential clash," said an Iraqi official who spoke on the condition of anonymity because of the sensitivity of the topic. "On the ground, [there are] tanks and armored vehicles. It's a very complicated situation with the army."
In a sign of US concern, President Barack Obama on Monday publicly announced his backing for Abadi, saying that his nomination was "a promising step forward". Both he and Vice President Joe Biden called Abadi to express their support.
Abadi now has 30 days to form a government, and during that time Maliki will remain the caretaker prime minister.
The US government has said it will significantly expand aid to Iraq in its battle with Al Qaeda-inspired militants only if it forms a government that embraces the country's different religious and ethnic groups.
"The only lasting solution is for Iraqis to come together and form an inclusive government," Obama told reporters in Martha's Vineyard, Massachusetts, where he is vacationing.
In his eight years as premier, Maliki has consolidated power in his office, ruling in an authoritarian style that has chipped away at his support among minority Sunnis, as well as his fellow Shiites. He is widely blamed for fostering an environment that has allowed Sunni extremists from the Islamic State to seize control of huge chunks of Iraqi territory.
But in the face of a chorus of calls for him to leave — from the country's religious authorities, his political rivals and even members of his own bloc — Maliki has stubbornly refused. Pressure to form a new government has mounted amid fears that Islamist extremists will exploit any vacuum. Maliki maintains, however, that since his bloc won the most seats in parliament in national elections this spring, he should be the one leading the next government.
Hours after Iraqi President Fouad Massoum asked Abadi to form a government, the enraged outgoing premier made a televised address lambasting the move and declaring that he personified the governing State of Law coalition.
"I am Nouri al-Maliki, and I am the head of State of Law, and I am the head of Dawa [party], and no one has the right to deal under our name without my approval," he said, in a speech that suggested his desperation.
But his State of Law coalition has crumbled, with 38 of its 96 parliamentarians signing a letter to the president declaring their support for Abadi. They were among 127 Shiite politicians who supported Abadi's bid in the 328-seat parliament. Abadi will probably be able to form a majority with support from Kurdish and Sunni factions, analysts said.
Indeed, a late-night show of force Sunday — when Maliki announced that he would sue the president rather than acquiesce to the naming of a new prime minister and deployed security forces to strategic points in the capital — appeared to have galvanized efforts to oust him.
"It has backfired and was unwise," said Hoshyar Zebari, a Kurd who served as foreign minister in the Maliki government. "We have passed the stage of military coups and taking power by force."
The army indicated Monday that its loyalties do not lie with Maliki.
"We are the army of Iraq, not of Maliki," the armed forces said on its official Twitter account. "We will continue to fulfill our promises, and for our nation we shall be defenders."
But Iraq's security forces are fragmented and in disarray after a partial collapse of the army in June.
Maliki has also built up a support base in Iraq's militias, most notably the notorious Asaib Ahl al-Haq, whose fighters were out in force for a second night Monday. However, analysts argue that Asaib and others are ultimately loyal to Iran, which has also withdrawn its backing for Maliki.
The Obama administration has deployed its top officials to encourage a smooth transition to a government led by Abadi, a seasoned member of Maliki's party.
In a phone call Monday morning with Massoum, Biden commended the Iraqi president for nominating the prime minister and emphasized Obama's "desire to boost coordination with a new Iraqi government and Iraqi Security Forces to roll back gains" by the Islamic State, according to a statement. So far, a US air campaign against the insurgents has largely been contained to the country's mostly Kurdish north.
In his speech Sunday night, Maliki accused the United States of supporting a breach in the Iraqi constitution.
Maliki on Monday filed a letter with Iraq's Supreme Court arguing that Abadi's appointment is null and void. His argument centers on a technicality: that his State of Law bloc, rather than a wider Shiite coalition, should have been allowed to nominate the president and that he is the only one who can speak for it. Human rights organizations have in the past complained that he controls the judiciary.
"They stabbed us in the back," Jumaa al-Adwani, a member of the State of Law coalition who is sticking by Maliki, said Monday, calling the move a coup.
Iraqi and US officials hope that a new leader will help bridge the country's rifts and bring Iraq's disaffected Sunni minority back into the political process, eroding support for the Sunni extremists.
Abadi, a British-educated engineer who hails from the capital, Baghdad, still faces an uphill battle.
"He's a fresh face in a sense," said Kirk Sowell, a political risk analyst and publisher of the newsletter Inside Iraqi Politics, who also described Abadi as a "gray suit kind of guy." "But this isn't a radical change, it's a modest change."
Abadi, a former communications minister, told state television that his first task as prime minister would be to curb the influence of the Islamic State.