Legislators in Mexico’s lower house of Congress on Wednesday morning approved a sweeping proposal to redesign the entire judiciary, the first step toward shifting the country to a system in which nearly every judge is elected to office.

The vote advances one of the most far-reaching judicial overhauls in any major democracy in recent decades, raising tensions in Mexico over whether the measures will improve the functioning of the country’s courts or politicize the judiciary in favor of the ruling Morena party and its allies. In the current system, judges are appointed based on special training and qualifications.

The lower house will now have to iron out more than 600 details of the bill before it moves to the Senate, where the ruling bloc is just one seat short of a supermajority — though the measure is expected to pass.

On Tuesday, when lawmakers met to discuss the initiative, eight of the 11 Supreme Court justices voted to suspend sessions for the rest of the week in support of striking judicial employees at the high court, who began a walkout earlier in the week — joining the hundreds of judicial workers and federal judges across Mexico who went on indefinite strike last month over the proposed changes.

In the hope of delaying the vote, striking workers formed a human chain to block access to Mexico’s lower house. But legislators switched venues to a sports complex and proceeded with the debate, which often turned into a tense exchange of accusations.

After an hourslong session that dragged on well into Wednesday morning, 359 lawmakers present voted in favor of the overhaul. Only 135 opposed it.

The results reflected the governing Morena party’s exceptional sway, which was achieved by landslide victories in a general election in June.

Members of Morena mobilized to approve the proposal at the bequest of the departing president, Andrés Manuel López Obrador, who is in the last month of his six-year term. His successor, President-elect Claudia Sheinbaum, has fully backed the plan.

“We went to the streets, we went knocking on doors, we went to towns,” Ricardo Monreal, Morena’s leader in the lower house, said during the debate, “and we told the people that if they voted for us we would vote for the reforms of President López Obrador. We did not deceive them, we did not deceive anyone.”

Mr. López Obrador’s plan has sparked recent protests across the country and has even been at the center of a diplomatic spat with the U.S. ambassador, Ken Salazar, who called it “a major risk to the functioning of Mexico’s democracy.”

The government says the proposed measures — which, among other things, would allow voters to elect nearly all of the country’s more than 7,000 federal, state and local judges — are needed to modernize the judiciary and instill trust in a system plagued by corruption, influence-peddling and nepotism.

Many critics of the plan agree that the system needs revamping. But they warn that the government’s proposal would do little to rid the judiciary of its problems. Instead, they say it would erode judicial independence and allow Mr. López Obrador’s political movement to concentrate power.

“Let’s say it loud and let’s say it clear,” said Patricia Flores, a lawmaker with the opposition Citizens’ Movement party, during the debate on Tuesday. “More than a judicial reform, this is an act of revenge — because the judiciary has been a counterweight to the decisions of the president.”

Although a few countries do allow the election of some judges by popular vote — including the United States, Switzerland and Japan — experts say none of them do it in such a sweeping way as the proposed changes would.

Mexico’s experiment, if approved, would shift the judiciary from an appointment system largely based on legal training and qualifications to one in which candidates with a law degree and relatively little experience could run to be elected as a judge.

“We’re going to see some very bad rulings at the beginning,” said Juan Jesús Garza Onofre, a constitutional law researcher at the National Autonomous University of Mexico. “There is going to be a learning curve that involves delaying processes that are already underway. They’re not going to take over the reins of the court overnight.” (Each of the more than 1,600 Mexican federal judges deals with, on average, 19 cases per week.)

Hamlet García Almaguer, a Morena legislator, said the bill calls for the creation of committees to evaluate aspirants’ skills and experience before they are allowed to run.

The bill would create a Tribunal for Judicial Discipline, also elected by popular vote, with broad powers to investigate, penalize and possibly fire or impeach judges — not including Supreme Court justices. The tribunal’s decisions would be final and not subject to appeal.

The tribunal, Mr. García Almaguer said, is needed because current disciplinary mechanisms have had little effect. “There is a mantle of impunity linked to nepotism and the relationships that exist there,” he said, adding that hundreds of judges and judicial workers have relatives working within the judiciary.

Critics of the overhaul, however, say that the tribunal or party leaders could put pressure on judges to deliver the rulings they want. “This would become a tool of political persecution,” said Judge Juana Fuentes, a member of the federal judiciary who opposes the plan. They could tell judges, she said, “You have to rule like this because, see, if you don’t, I’ll remove you.”