Vietnam’s top leader, To Lam, has asked President Trump to delay the imposition of tariffs for at least 45 days so the two sides can avert a move that would devastate the Vietnamese economy and raise prices for American consumers.

The 46 percent tariff rate the United States has said it will impose on Vietnam is among the highest any country faces. The prospect of such a steep tariff has left Vietnam with a sense of whiplash and deep apprehension. It also presents a sharp contrast to Washington’s recent embrace of Hanoi as an important bulwark against China and a manufacturing destination for many American apparel brands.

Mr. Lam’s proposal to President Trump was laid out in a letter dated Saturday, according to a copy obtained by The New York Times. In the letter, Mr. Lam called on Mr. Trump to appoint a U.S. representative to lead negotiations with Ho Duc Phoc, a Vietnamese deputy prime minister, “with the goal of reaching an agreement as soon as possible.”

Mr. Lam had been one of the first world leaders to reach out to Mr. Trump after the tariffs were announced. In a phone call, he offered to reduce tariffs on U.S. imports to zero, and urged Mr. Trump to do the same, according to the Vietnamese government. Vietnam has said its tariffs on U.S. goods is an average of 9.4 percent.

Mr. Trump later described the call as “very productive.”

In his letter, Mr. Lam asked Mr. Trump to meet him in person in Washington at the end of May “to jointly come to an agreement on this important matter, for the benefit of both our peoples and to contribute to peace, stability and development in the region and the world.”

Vietnam’s Foreign Ministry did not respond to a request for comment.

Vietnam, which faces punishingly high tariffs along with China, Cambodia and Laos, would be the hardest-hit economy in Asia if the tariffs are imposed as planned on Wednesday, according to economists. The United States is Vietnam’s largest export market, accounting for about 30 percent of the country’s total exports. A 46 percent tariff rate would put 5.5 percent of Vietnam’s gross domestic product at risk, according to ING, a Dutch financial services company.

It would also hurt American consumers, because Vietnam is crucial in the global manufacturing supply chain. For decades, the country has built its economy around attracting foreign investment with cheap labor and a young work force. It is now a top manufacturer of brands such as Adidas and Lululemon. Nike makes about 50 percent of its footwear in Vietnam.

After Mr. Trump imposed tariffs on China during his first term, Vietnam benefited from companies shifting their manufacturing there.

Within Hanoi, the recent moves by the Trump administration have cast doubts on the reliability of the United States, which in recent years has assiduously courted Vietnam. In 2023, the two former enemies cemented a new strategic relationship, a move seen as a milestone in U.S. foreign policy. Even though Vietnam fought a brutal, decades-long war against the United States, surveys had shown that many Vietnamese welcomed the political and strategic influence of the United States.

The Biden administration viewed Vietnam — one of the few Southeast Asian nations that has publicly pushed back against China’s assertiveness in the South China Sea — as critical to the U.S. effort to counter China’s mounting ambitions in the region.

“Vietnam’s position in the Pacific, its view on China, its willingness to work with America, was its strongest card,” said Huong Le Thu, the deputy program director for Asia for the International Crisis Group. “Trump doesn’t see it that way. He doesn’t see allies or strategic values. He just sees numbers and tariffs, so Vietnam needs to try harder.”

Analysts say Vietnam had a largely positive opinion of Mr. Trump during his first administration, seeing him as a pragmatic businessman who would not moralize with them over human rights.

While explaining the tariffs, Mr. Trump said: “Vietnam: great negotiators, great people, they like me. I like them.” He said “the problem” was that the country charges the United States “90 percent,” a figure apparently reached by basing it on Vietnam’s current trade surplus with the U.S., worth $123.5 billion. (Vietnam disputed that calculation.)

The tariffs come at a precarious time for Mr. Lam, who was named as general secretary of Vietnam’s ruling Communist Party last year after the death of the previous party chief, Nguyen Phu Trong. Mr. Lam needs to secure a strong economic performance as he heads into next year’s party congress, where the country’s top leaders will be selected.

Even before Mr. Trump’s tariff announcement, Vietnam was working to win favor with the new administration. It signed provisional deals to import U.S. liquefied natural gas, cut some tariffs on American imports, and allowed SpaceX to open a company to launch its Starlink satellite internet service in Vietnam. The Trump organization is developing a $1.5 billion golf course and hotel project in Vietnam’s northern Hung Yen province, Mr. Lam’s home province.

Alexandra Stevenson contributed reporting.