France is heading to the polls on Sunday for the first round of snap legislative elections that President Emmanuel Macron unexpectedly called this month, a gamble that has thrust the country into deep uncertainty over its future.

Voters are choosing their 577 representatives in the National Assembly, the country’s lower and more prominent house of Parliament, which will determine the future of Mr. Macron’s second term.

A new majority of lawmakers opposed to Mr. Macron would force him to appoint a political opponent as prime minister, radically shifting France’s domestic policy and muddling its foreign policy. If no clear majority emerges, the country could be headed for months of turmoil or political deadlock. Mr. Macron, who has ruled out resigning, cannot call new legislative elections for another year.

France’s nationalist, anti-immigrant National Rally party is widely expected to dominate the race. A broad alliance of left-wing parties could come in second. Mr. Macron’s centrist Renaissance party and its allies are expected to lose many seats.

Most polls will close at 6 p.m. local time on Sunday, or as late as 8 p.m. in larger cities. Nationwide voting projections provided by polling institutes, based on preliminary results, are expected right after 8 p.m. and are usually reliable. Official results, published by the Interior Ministry, will come in throughout the night.

Here is what to expect.

The voting happens in two rounds, and participation is expected to be high.

France’s 577 electoral districts — one for each seat — cover the mainland, overseas departments and territories, and French citizens living abroad. In each district, the seat is awarded to the candidate who gets the most votes.

Any number of candidates can compete in the first round in each district, but there are specific thresholds to reach the second round, which will be held a week later, on July 7.

In most cases, the second round features the top two vote-getters, and whoever wins the most votes in that runoff wins the race. But there are exceptions.

A candidate who gets more than 50 percent of the vote in the first round wins outright, as long as those votes account for at least a quarter of registered voters in that district. And the runoffs in some districts could feature three or even four candidates if they are able to get a number of votes equal to at least 12.5 percent of registered voters.

Both scenarios have been rare in past years, but they are more likely if voter abstention is low, as is expected on Sunday. Most polling institutes expect the voter participation rate to exceed 60 percent in the first round, compared with 47.5 percent in 2022.

As of noon on Sunday, the participation rate was nearly 26 percent, the Interior Ministry said, higher than in 2022, when it was only 18.43 percent at noon.

France’s legislative elections typically occur just weeks after the presidential race and usually favor whichever party has just won the presidency, making the elections less likely to draw in voters who feel like the outcome is preordained.

But the stakes are much higher this time.

A surging far right, a strong left-wing alliance and a shrinking center are on display.

The goal for each party and its allies is to get enough seats to form a working majority. If none of them do, France may face months of political turmoil or gridlock.

But if control of the National Assembly flips over to Mr. Macron’s opposition, he would be forced to appoint a prime minister and cabinet of a different political party, which would then control domestic policy. Presidents traditionally retain control over foreign policy and defense matters in such scenarios, but the Constitution does not always offer clear guidelines.

The National Rally has a comfortable lead in the latest polls, with the support of roughly 36 percent of voters. After decades on the fringes, the anti-immigrant, euroskeptic far right has never been closer to governing France, which would be a stunning development in a country that has been at the heart of the European project. A National Rally prime minister could clash with Mr. Macron over issues like France’s contribution to the European Union budget or support for Ukraine in its war against Russia.

The alliance of the Socialists, Greens, Communists and hard-left France Unbowed party has been polling in second place, with about 29 percent support, and it believes it has a chance to overcome the far right and form a government of its own. The alliance wants to overturn some of what Mr. Macron’s government did over the past seven years, like raise the legal age of retirement. It also wants to roll back corporate tax cuts and tax breaks for the rich to vastly increase social spending, and pass a big minimum wage hike.

For Mr. Macron’s centrist party and its allies, the contest is an uphill battle. The polls put them in third place, with roughly 20 percent, and widely predict them to lose many of the 250 seats they hold. Some of Mr. Macron’s political allies are running — the leaders of other centrist parties, some of his own ministers and even the prime minister — and defeats for any of them would be a blow.

First-round results could give an imperfect sense of where the vote is headed.

In 2022, Mr. Macron’s centrist coalition and the left were neck and neck in the first round of voting, ahead of all other parties, with roughly a quarter of the vote each. A week later, both were still ahead of the competition — but Mr. Macron’s coalition won nearly 250 seats, and the left secured fewer than 150.

In other words, while the first round of voting is an indicator of what the final results might be, it is not a perfect predictor.

One way to analyze the first round is to look at nationwide voting trends: What percentage of the vote did each party get around the country? This is a good way to see whether polling accurately predicted the general popularity of each party, and to see which forces have momentum for the final week of campaigning.

But nationwide voting percentages obscure the fact that France’s legislative elections are, in essence, 577 separate races, and most seats are decided only after the second round.

Each party’s prospects depend on how many runoffs their candidates are in — the more they reach, the stronger their party’s chances of coming ahead on July 7. What kind of matchups they will face will also become clearer.

And a lot happens between the two rounds. Voters whose favored candidates do not make it into the runoff will either shift to another, or just stay home.

Parties will issue local or nationwide voting recommendations to try to influence the outcome. In the past, parties across the spectrum often appealed to their members to vote strategically against the far right, but that tactic has frayed.

Candidates can decide to withdraw from a three- or four-way race if they worry about splitting the vote; several left-wing parties have already announced that they would encourage their candidates to do so.

There will also be a new week of campaigning — more than enough time for gaffes, missteps or twists that could change the course of any race.