French President Emmanuel Macron’s gamble to hold a snap election to thwart the power of rising right-wing parties is looking ever more risky as final polls showed the far-right National Rally poised to score major gains in the first of two rounds of voting Sunday.

One poll, conducted for the newspaper Los Echos and released Friday, shows the anti-immigration, euro-skeptic party taking 37% of the vote, up two percentage points, while a coalition of leftist parties was at 28% and Mr. Macron’s centrist Together bloc was trailing with just 20%.

Mr. Macron was betting that French voters in the second round of parliamentary voting set for July 7 would coalesce as in the past to keep the National Rally from building on its first-round gains, but a second poll done for BFM TV projected that the National Rally has an outside chance of winning the 289 seats in the national parliament that would give it an outright majority and the right to name a prime minister under Mr. Macron.

Even in a hung parliament, the far-right party looks almost certain to win the largest single bloc of votes and make life difficult for Mr. Macron through the end of his term in 2027.

The polls are the last snapshot analysts will have before Sunday’s vote. Campaigning for the first round officially ends at midnight Friday.

Longtime National Rally leader Marine Le Pen has worked hard to make the party more acceptable to a broader slice of the French electorate, and she has benefited from deep voter unhappiness with the state of the economy and public services under Mr. Macron. Like other surging right-wing parties in Europe, Ms. Le Pen and her aides have promised much stricter immigration laws and a get-tough approach to crime and public safety.

Mr. Macron and the coalition of leftist parties are expected to coordinate their campaigns in the second round of voting, in a bid to deny the National Rally more seats.

The short, three-week campaign — which Mr. Macron sought after the National Rally won big in the June 9 elections for seats in the European parliament — has proven too brief to turn around the political dynamic, Brice Teinturier, deputy director of the polling firm Ipsos, told Le Monde daily.

The National Rally bloc, he said, “is incredibly powerful.”

• This story is based in part on wire service reports.

• David R. Sands can be reached at dsands@washingtontimes.com.