BAGHDAD — Iraqi political leaders spent the last 10 months struggling unsuccessfully to form a government, their country sinking deeper and deeper into political paralysis in the face of growing drought, crippling corruption and crumbling infrastructure.

Then in June, those talks imploded. And now, there is a scramble for power as Iraq’s main political factions vie for the upper hand.

The powerful Shiite Muslim cleric Muqtada al-Sadr, who leads the largest bloc in Parliament, quit the negotiations in frustration, then urged his followers to take to the streets to get what they wanted. Heeding his call, they set up a tent encampment that has blocked access to Parliament for more than two weeks to prevent any government from being voted in.

It is not the first time that Mr. al-Sadr has resorted to the threat of violence to get what he wants politically. He led the armed Shiite revolt against the American occupation of Iraq from 2003 to 2009, and U.S. officials say they now worry that Iraq could plunge again into violence and instability.

Equally alarming, despite years of American efforts to shape Iraq into an alternative Shiite power center that would be more Western-oriented than Iran, ‌Mr. Sadr and his Shiite political rivals favor a political system that would confer more power on religious clerics along the lines of an Iranian-style theocracy.

“We’re looking at the beginning of the end of the American-backed political order in Iraq,” said Robert Ford, a former American diplomat in Iraq and now a fellow at Yale University and the Middle East Institute.

For decades, Iraq has reeled from crisis to crisis — a cycle that shows no signs of abating. After the 2003 U.S. invasion to oust Saddam Hussein, there was a civil war, and then the takeover of large parts of the country by the Islamic State.

As a result, Iraq, despite vast oil reserves, has remained mired in political chaos with a stagnant economy that has left its unemployed youth vulnerable to recruiters for extremist movements and made investors leery. At the same time, Gulf States led by the United Arab Emirates normalized relations with Israel and forged ahead politically and economically to become the new center of gravity of the Middle East.

And the U.S. vision for Iraq’s future has seemed to slip further and further away.

When President George W. Bush invaded in 2003, his government tried to encourage Iraqi political leaders to set up a representative system that would share power more equitably among the country’s three main groups — the Shiite majority, and the Sunni Muslim and Kurdish minorities.

“The Americans were kind of hoping that there would be these cross-sectarian and more policy-centered alliances between the political factions, but the sectarian and ethnic divisions won out,” Mr. Ford said. “Instead, we have this squabbling between and within sectarian and ethnic communities about how to divide Iraq’s oil money.”

About 85 percent of the Iraqi government is funded by oil income, according to the World Bank. And under the current political system, each major political faction in Parliament gets control over at least one government ministry, and with it, patronage jobs and the opportunity to skim money and pocket kickbacks.

As politicians have focused more on their own power than national interests, Iran has found it easier to persuade a number of Sunni, Kurd and Shiite leaders to support the policies it cares most about: the cross-border movement of Iranian arms, people and goods.

The crisis now enveloping Iraq pits Mr. Sadr and his mostly Shiite supporters against a coalition of Shiite parties with militias linked to Iran in a bitter power struggle. The caretaker government, fearing violence, has been reluctant to disrupt Mr. Sadr’s blockade, allowing him to hold the country hostage to a sweeping list of demands: the dissolution of Parliament, new elections, and changes in election law and possibly the Constitution. However, on Tuesday, Prime Minister Mustafa al-Kadhimi invited all the parties to the government palace “to find solutions to the current political crisis.”

“It looks like a peaceful coup d’état, a peaceful revolution,” Mahmoud Othman, a former Parliament member who was not affiliated with any political party, said of the Sadrists’ blockade of Parliament. “I say peaceful, because his followers are not carrying guns. Sadr is stronger than guns. He is now the strongman on the street, and he is imposing his will on others.”

So far the blockade has not been violent.

Several thousand Sadrists occupy the tent encampment, working in shifts. They wander about, listening to clerics denounce government corruption and eating shawarma, grapes and watermelon donated by sympathizers. They rest in tents in the heat of the day, waiting for Mr. Sadr’s next instructions via tweet — his favored means of communication.

Sunnis and Kurds have remained on the sidelines.

Many Sunnis say they feel disenfranchised and see no role for themselves in the future Iraq, and many wonder whether it would be better to divide the country and have a separate Sunni enclave, said Moayed Jubeir Al-Mahmoud, a political scientist at the University of Anbar in the city of Ramadi, a Sunni stronghold.

“Unfortunately I do not see a secure and prosperous future for my country,” he said, describing Iraq as a failed state controlled by Iran-linked militias. “We are concerned that the state will just go from being dominated by militias to being dominated by al-Sadr.”

The United States and most neighboring countries have stayed largely silent about the chaos in Iraq. Only Iran has tried to intervene, meeting with Mr. Sadr’s Shiite opponents and encouraging negotiations even though Mr. Sadr, a nationalist, has taken a strongly anti-Iranian stance in recent years.

The last thing Iran wants is for Shiites to fight one another and risk weakening their grip on power, which could end up undercutting Tehran’s influence in Iraq.

A number of Mr. Sadr’s positions align with Tehran. Both want to force the remaining 2,500 U.S. troops to leave Iraq, oppose any interactions with Israel and favor criminalizing homosexuality.

Mr. Sadr has resorted to mass demonstrations before, but this time he is using street protests to force the country to ignore last October’s election results and to hold a new vote that could return his legislators to power.

The parliamentary election 10 months ago went well for Mr. Sadr. Legislators who supported him won the most seats of any faction and had almost forged a governing coalition supported by Kurdish and Sunni partners. The next step would have been to bring it to a vote for approval.

Mr. Sadr’s Shiite rivals, however, refused to attend the Parliament session, denying him the quorum needed for a vote. Frustrated, Mr. Sadr asked his legislators to resign in protest.

The parties who had gotten fewer votes, primarily his Shiite rivals, then filled the seats that Mr. Sadr’s followers had vacated, potentially giving them control over ministries and government offices and leaving Mr. Sadr out.

He responded by calling for the blockade of Parliament to prevent a vote on a new government.

“So this is when Muqtada al-Sadr decided that if the democratic procedures are not allowed to play themselves out, then the response is revolution,” said Rend Al-Rahim, a former Iraqi ambassador to the United States and the president of the Iraq Foundation, a nonprofit organization that promotes democracy.

At the tent encampment, the atmosphere is decidedly Shiite. Last week, Mr. Sadr’s followers marked Ashura, which commemorates the death of Hussein, a grandson of the Prophet Muhammad. His death is often depicted as the start of the division between Sunni and Shiite Muslims.

Everywhere there were signs of support for Mr. Sadr’s cause: Even some of the poorest chipped in to pay for a tent or meals. A water company donated enough every day to fill the large tanks that supply the tent dwellers. The markets in Sadr City — a poorer area of Baghdad filled with Sadr loyalists — sent crates of tomatoes, onions, dates, grapes and apples.

To cope with the 115-degree heat in daytime, some protesters installed large fans or air coolers hooked up to Parliament’s 24-hour electricity supply.

“It’s the first time we have had electricity 24 hours a day,” said Faiz Qasim, an enthusiastic Sadr organizer who usually works as a day laborer. Much of Baghdad suffers from daily electricity cuts.

Sadr supporters from the south of Iraq prepared large caldrons of stews daily. One day it was a rich curried chicken, while nearby, the next day’s meal — a black-and-white cow tethered to a cellphone tower — placidly masticated some watermelon. A little further down the same street, another cow was being slaughtered for dinner that night.

Clerics periodically rallied groups of men — there are almost no women in the tents — with chants against the current political leaders:

Many people suffered from those who were here in this swamp.
They climbed to power on the backs of the innocent and Iraq suffered because of them.
There are many people holding out their hands, begging in the streets and going through the garbage.
Al-Sadr says America and Israel have the money and the weapons. But what do we have?
Allah almighty.

Falah Hassan contributed reporting.

By Alissa J. Rubin / The New York Times