PARIS — French voters face a decisive choice Sunday in the runoff of snap parliamentary elections that could produce the country’s most right-wing government since the World War II Nazi occupation — or no majority emerging at all.

The National Rally, an anti-immigration, nationalist party led by Marine Le Pen, stands a chance of winning a legislative majority for the first time, but the outcome remains uncertain because of a complex voting system and tactical maneuvers by political parties.

Voters across France and overseas territories can cast ballots for 501 of the 577 seats in the National Assembly, the lower and most important of France’s two houses of parliament. The other 76 races were won outright in the first round of voting.

The National Rally and its allies arrived ahead in Round One with around one-third of the votes. A coalition of center-left, hard-left, and green parties called the New Popular Front came in second position, well ahead of President Macron’s struggling centrist alliance.

In the frantic week between the two rounds, more than 200 centrist and left-wing candidates pulled out of races to boost the chances of their moderate rivals and try to keep National Rally candidates from winning.

Final pre-election polls suggest that tactic may have diminished the far right’s chances of an absolute majority. Madam Le Pen’s party has wider and deeper support than ever before, and it’s up to voters to decide.

Polling projections suggest the National Rally is likely to have the most seats in the next National Assembly,  which would be a historic first.

If it wins an absolute majority of 289 seats, Mr. Macron would be expected to appoint National Rally president Jordan Bardella as France’s new prime minister. Mr. Bardella could then form a government, and he and Mr. Macron would share power in a system called “cohabitation.’’

If the party doesn’t win a majority but still has a large number of seats, Mr. Macron could name Mr. Bardella anyway, though the National Rally might refuse out of fears that its government could be ejected in a no-confidence vote.

Or Mr. Macron could seek to build a coalition with moderates and possibly choose a prime minister from the center-left. If there’s no party with a clear mandate to govern, Mr. Macron could name a government of experts unaffiliated with political parties. Such a government would likely deal mostly with day-to-day affairs of keeping France running.

Complicating matters is the fact that any of those options would require parliamentary approval. If political talks take too long amid summer holidays and, between July 26 and August 11 the Olympics at Paris, Mr. Macron’s centrist government could keep a transitional government pending further decisions.

If an opposition force wins a majority, Mr. Macron would be forced to appoint a prime minister belonging to that new majority. In this “cohabitation,” the government would implement policies that diverge from the president’s plan. 

France’s modern Republic has experienced three cohabitations, the last one under a conservative president, Jacques Chirac, with a Socialist prime minister, Lionel Jospin, from 1997 to 2002.

The prime minister is accountable to the parliament, leads the government and introduces bills. The president is weakened at home during cohabitation, but still holds some powers over foreign policy, European affairs, and defense and is in charge of negotiating and ratifying international treaties. The president is also the commander-in-chief of the country’s armed forces, and holds the nuclear codes.

While not uncommon in other European countries, modern France has never experienced a parliament with no dominant party. Such a situation requires lawmakers to build consensus across parties to agree on government positions and legislation.

France’s fractious politics and deep divisions over taxes, immigration and Mideast policy make that especially challenging. That would likely derail Mr. Macron’s promises to overhaul unemployment benefits or legalize life-ending procedures for the terminally ill, among other reforms. It could also make passing a budget more difficult.