The Wall Street Journal / Sam Dagher

BAGHDAD—A barrage of at least 15 bombings left dozens dead across the Iraqi capital Thursday, in a violent escalation of a conflict that threatens to rip apart the country's delicate political fabric just days after the departure of U.S. troops.

How Iraqis resolve the dramatic resurgence of their deep sectarian and ethnic divisions could determine whether the nation holds together in the form the U.S. tried to steward over the past nine years.

At the center of the conflict is Prime Minister Nouri al-Maliki, whose Shiite-dominated government has quickly become enmeshed in battles with Sunni provinces and officials—including seeking the arrest of the vice president—even as Iraq's third major interest group, the Kurds, tries to assert more autonomy.

Mr. Maliki is working to retain political control and national security in the wake of the official completion of the U.S. troop withdrawal on Dec. 18, but is being assailed by the other groups for stoking the conflict. The bombings on Thursday, which rocked almost every major neighborhood in the capital and left at least 59 people dead, seemed designed to undermine Mr. Maliki's government.

The Obama administration said it believed the attacks were planned "over a period of time and not necessarily tied to the events of the past week," said Mark Toner, the State Department spokesman.

But Mr. Maliki said political rivals were behind the bombings and avoided mention of the insurgents typically blamed in previous attacks, in a statement that could further fan sectarian tensions. Two of the prime minister's opponents warned against attempts to "sow the seeds of sectarian discord."

Iraq's factionalized political elite has already plunged into a high-stakes showdown. The long-running conflict splashed into a public feud just hours after the last U.S. troops exited Iraq Sunday. It was then that Mr. Maliki's government launched a public campaign accusing Vice President Tariq al-Hashemi, a prominent Sunni leader, of organizing the assassinations of Shiites, including government figures.

Mr. Hashemi and his supporters say the charges—made by a judiciary Mr. Maliki says is operating independently but his critics charge is in his grip—are smears designed to weaken Mr. Maliki's foes and strengthen his own position. Mr. Hashemi is a senior member of Sunni Arab-dominated Iraqiya, the biggest opposing political faction, which one year ago reluctantly agreed to join Mr. Maliki's shaky coalition government.

The faceoff follows efforts by two Sunni-dominated provinces to initiate the legal process of gaining greater autonomy from Baghdad—moves that were quickly quashed by Mr. Maliki.

Mr. Hashemi fled this week with several other members of Iraqiya to the country's north, where he is receiving protection in Kurdistan, an area controlled by Iraqi Kurds. Mr. Hashemi has supported moves toward autonomy for Sunni provinces.

The Kurds, mostly Sunni Muslims who are ethnically distinct from the Arabs who dominate the rest of Iraq, find themselves once more in the position of exploiting sectarian divisions among Arabs.

The Kurds also have a stake in the political conflict. They seek to maintain, and expand, their virtual state-within-a-state in northern Iraq, which they have built largely beyond the central government's control. Both sides have long been at loggerheads over a law that would govern how oil revenues are to be shared in the country.

U.S. officials have pressed Iraqi leaders to overcome their differences. On Thursday, Vice President Joe Biden called Iraqi President Jalal Talabani to offer support for his efforts to foster dialogue, the White House said.

U.S. officials in Washington said they were alarmed at the sudden bloodshed. A dozen attacks during the morning rush hour targeted Shiite, Sunni and mixed neighborhoods, hitting mostly civilian targets, including an elementary school.

The worst involved a suicide bomber driving an ambulance packed with explosives that blew up in front of a government office in the central district of Karrada, a mixed commercial and residential area inhabited mostly by Shiites and Christians. There, 13 people were killed and 36 wounded, according to the Ministry of Interior.

Another attack, involving two improvised explosive devices and a car bomb, targeted day laborers waiting for work at the busy Alawi transportation terminal in central Baghdad.

Later, most roads were closed as the army and police cordoned off the numerous blast sites. Sirens of emergency vehicles were heard throughout the day.

The violence came as a longstanding political struggle comes to a head. Sunni Arabs and Kurds worry the allegations against Mr. Hashemi could herald a return to the repressive autocracy of the Saddam Hussein era, this time under Shiite control at Mr. Maliki's hands.

Many Sunni leaders, such as Parliament Speaker Osama al-Nujaifi, who were once among the staunchest supporters of a strong central state, are now convinced that the only hope for survival in a Shiite-ruled Iraq is carving out a semiautonomous Sunni region, as allowed by the Iraqi Constitution. "The solution for Iraq's worsening problem is the formation of regions," said Mr. Nujaifi earlier this month.

Mr. Maliki, who has projected himself as a Shiite Arab nationalist who will do anything to preserve Iraq's unity, decries moves toward greater regional autonomy. "This is a recipe for partition," he told state media last week on a flight back from Washington, after predicting a smooth transition following the end of the U.S. military mission.

"Federalism as an idea has been a grenade waiting to have the pin pulled from it ever since the Constitution of Iraq, passed in 2005, allowed for the formation of 'regions,' " wrote Gareth Stansfield, a researcher at Chatham House, in a recent report.

While each side in the conflict accuses the other of resorting to naked power plays, they also accuse each other of aligning with foreign countries—such as Shiite Iran with Mr. Maliki or Sunni Saudi Arabia and Turkey in the case of Iraqiya—in a battle for influence that is intensifying in the wider Middle East.

Iraq's post-Hussein political structure, a flawed and fragile combination of elections and ethnic and sectarian quotas, has largely kept the country functioning through some of the darkest hours since the U.S.-led invasion. It has managed to balance intense influence by neighboring states with the U.S. presence.

While dissatisfaction is widespread—electricity supply remains a challenge, nearly a quarter of Iraqis are unemployed and a similar share live below the poverty line, according to United Nations and World Bank figures—a measure of stability in recent years has brought gains.

Gross domestic product is expected to grow by 9.6% this year, according to the International Monetary Fund. The government is forging ahead with an ambitious, and costly, plan to quadruple oil production by 2017. Despite the political and security risks, foreign companies have been racing in.

The central government's draft budget for next year is $100 billion, of which nearly $32 billion has been earmarked for investment spending. But implementing plans could prove disastrous while Baghdad and the provinces are squabbling and sectarian tensions are high, warns Ghassan Atiyyah, a political analyst and consultant to a number of Iraqi leaders.

The federalism provision of Iraq's Constitution was crafted to formalize the Kurds' de facto control of the northern part of Iraq. But as the U.S. withdrawal loomed, Sunni-dominated provinces, with large populations unhappy with Shiite-dominated governments, moved to form their own regions. Most Iraqis look with envy at the relative stability and oil-fueled construction boom in the Kurdish region, which has yielded reliable public services compared with the dysfunction in the rest of the country.

The province of Salahuddin moved in October to form its own region. Its provincial council, based in the Sunni stronghold of Tikrit, sees autonomy as a way for the population of 1.3 million to carry out its own infrastructure projects, cut through central government red tape and keep the majority of revenues from any oil or gas discoveries.

Ammar Youssif Hammoud, head of the provincial council, also sees autonomy as an escape from what he views as an increasingly authoritarian central government. "Look at this so-called Arab Spring. People everywhere have revolted against dictatorial and centralized regimes," he said.

Mr. Maliki's government has refused to begin planning a referendum for Salahuddin's request as outlined under the constitution, citing legal objections.

Diyala, a volatile majority Sunni province with a mix of Shiites and Kurds in eastern Iraq, announced plans last week to become a region. Anbar, a Sunni-dominated province in western Iraq, is poised to be next.

Mr. Maliki moved quickly last week to crush Diyala's bid through protests and road closures led by Shiites under the protection of security forces beholden to the prime minister.

Kurdish leaders back the Sunni push for autonomy in the hope it will force through an oil law to provide a legal framework for the more than 40 contracts it signed with energy companies in defiance of Baghdad. Also in the balance is the fate of disputed territories, most notably the oil-rich province of Kirkuk.

"This centralism is killing and paralyzing everything," says the Kurdish governor of Kirkuk, Najmaldin Karim, who favors the partition of Iraq into three or more regions, with Baghdad becoming a "federal zone."

Yet Kurdish leaders insist it isn't their regional aspirations that threaten to weaken Iraq.

"The Kurds will not be the ones who divide Iraq, it will be the Shiite-Sunni power struggle," says Barham Salih, prime minister of the Kurdistan regional government. "Whether this will go through to the full-blown dismemberment of Iraq remains to be seen."

Write to Sam Dagher at sam.dagher@wsj.com