Labor unions in France are carrying out a series of nationwide marches and strikes against President Emmanuel Macron’s unpopular plans to raise the legal age of retirement to 64, from 62.

On Tuesday, they vowed to bring France “to a standstill” with the sixth day in a string of protests over the past two months. Some of the demonstrations have gathered over a million people and they have emerged as a major test for Mr. Macron after his re-election last year, as public opinion polls have repeatedly shown that a majority of French people oppose his proposal.

After tense debates in the National Assembly, France’s lower house of Parliament, lawmakers in the Senate, the upper house, are discussing the bill, and the government hopes it will pass by the end of the month.

Mr. Macron and his government say they need to change France’s pension system to put it on a firmer financial footing as life expectancy rises and as the ratio of workers to retirees decreases.Opponents, including a united front of labor unions, dispute the need for urgency. They say that Mr. Macron is attacking a cherished right to retirement and unfairly burdening blue-collar workers because of his refusal to increase taxes on the wealthy.

Neither side has shown any sign of backing down.

Overall disruptions have been limited so far, with protests and intermittent walkouts by workers in schools, public transportation, fuel refineries and other sectors.

The government has made small concessions to shore up political support from right-wing lawmakers but is taking a harder line against the unions, which have organized massive demonstrations but have little to show for them.

Some unions are now calling for continuous strikes that could alienate public opinion but would turn up the pressure on Mr. Macron. He has said little publicly about the pension overhaul and left his government on the front lines of defending it. But he has staked much of his second-term legacy on getting it done.

Wait, why does this seem familiar?

The prospect of a pension overhaul has long been a third rail of French politics, prompting large protests in 1995 and 2010, long before Mr. Macron took office. This is the second time that Mr. Macron’s pension plans have met fierce resistance.

In 2019, during his first term, Mr. Macron’s effort to overhaul France’s generous pension system led to huge street protests and grinding strikes, including one of the longest transportation walkouts in the country’s history. The government shelved those plans after the coronavirus pandemic hit.

There is a key difference between what Mr. Macron did back then and what he is doing now: Mr. Macron’s initial project did not involve increasing the legal age of retirement. Instead, he was aiming for an across-the-board overhaul of the pension system’s dizzyingly complex architecture. The goal was to merge 42 different pension programs into what he said would be a fairer, unified system, using points that workers would accumulate and cash in upon retirement. But the plans left many confused and worried that their pensions would decrease.

So what is Macron doing this time?

The latest plans are a much more straightforward attempt to balance the system’s finances by making the French work longer, an effort that the government acknowledges will be difficult for some but that it insists is necessary.

France’s pension system relies primarily on a pay-as-you-go structure in which workers and employers are assessed mandatory payroll taxes that are used to fund retiree pensions. That system, which has enabled generations to retire with a guaranteed, state-backed pension, will not change.

France has one of the lowest rates of pensioners at risk of poverty in Europe, and a net pension replacement rate — a measure of how effectively retirement income replaces prior earnings — of 74 percent, according to the Organization for Economic Cooperation and Development, higher than the O.E.C.D. and European Union averages.

But the government argues that rising life expectancies have left the system in an increasingly precarious state. In 2000, there were 2.1 workers paying into the system for every one retiree; in 2020 that ratio had fallen to 1.7, and in 2070 it is expected to drop to 1.2, according to official projections.

Antoine Bozio, an economist at the Paris School of Economics, said that there was no short-term “explosion of the deficit” that needed to be addressed urgently. But “once you’ve said that the system isn’t in danger or on the verge of a catastrophe,” he said, “that doesn’t mean there isn’t a problem” in the long term.

To keep the system financially viable without funneling more taxpayer money into it — something the government already does — Mr. Macron wants to gradually raise the legal age of retirement by three months every year until it reaches 64 in 2030. He also wants to accelerate a previous change that increased the number of years that workers must pay into the system to get a full pension.

Mr. Macron has called the overhaul “indispensable.”

Why is the plan so unpopular?

Opponents say that Mr. Macron is exaggerating the threat of projected deficits and refusing to consider other ways to balance the system, like increasing worker payroll taxes, decoupling pensions from inflation or increasing taxes on wealthy households or companies.

Making people work longer, opponents argue, will unfairly affect blue-collar workers, who often start their careers earlier and who have a shorter life expectancy, on average, than white-collar ones.

“Sixty-four isn’t possible,” Philippe Martinez, the head of the CGT labor union, France’s second-largest, told French television in January. “Let them visit a textile factory floor, or a slaughterhouse, or the food-processing industry, and they will see what working conditions are like.”

Some worry about being forced to retire later because older adults who want to work but who lose their jobs often deal with age discrimination in the labor market.

The plan’s unpopularity also has much to do with pre-existing anger against Mr. Macron, who has struggled to shake off the image of an out-of-touch “president of the rich.”

By making pensions a cornerstone of his second term — he cannot run for a third consecutive one — Mr. Macron has also made them a referendum of sorts over his legacy, and analysts say that he could become a lame-duck president if he fails to pass the pension overhaul.

“That’s why he has not only all the unions, but also a large part of public opinion against him,” said Jean Garrigues, a leading historian on France’s political culture. “By tying himself to the project, opposition to it is heightened, dramatized in a way.”

What comes next?

The government has announced measures intended to mollify opposition, like continued exemptions allowing those who begin working at younger ages to retire earlier and measures to help seniors stay employed. The government also said it would increase smaller pensions, but that backfired after officials acknowledged that for most retirees, the bump would be weaker than initially announced.

But those concessions were mostly offered as carrots to garner the support of Republican Party lawmakers — mainstream conservatives whose votes the government needs to pass the bill — and they have not placated the unions.

Some unions are now calling for longer, more disruptive strikes in key sectors like transportation and energy to force the government’s hand.

Once the Senate votes on the bill, which it is expected to do by Sunday, a joint committee of lawmakers from both houses will work on reconciling their versions of the bill. Each house will then take a final vote.

But Mr. Macron’s party, Renaissance, and its allies no longer enjoy an absolute majority in the National Assembly, and would struggle to pass it on their own. They will have to rely on the Republicans, whose leaders have said they could support the bill.

Mr. Macron is hoping to weather the protests, much like Nicolas Sarkozy did as president in 2010, when he raised the retirement age to 62 from 60 despite huge demonstrations.

But some rank-and-file Republicans — and even members of Mr. Macron’s party — have expressed discomfort with the current version of the bill, meaning the vote could go down to the wire if protests from constituents push individual lawmakers to back out.

The government could use a rare constitutional tool to ram the bill through without a vote — Prime Minister Élisabeth Borne used this tactic several times in the fall to enact finance measures. But the procedure would expose the cabinet to a no-confidence motion. And using it on such a contentious and consequential piece of legislation could further inflame tensions on the streets.

Constant Méheut contributed reporting.