The letters were reviewed by The New York Times; they have not previously been reported.

In the most recent letter, dated May 2, Mr. Trump cites the 1996 and 2002 Olympic Games and the 1994 World Cup as examples of major international events hosted by the United States, and he assures Mr. Infantino — and by extension FIFA voters — that “I am confident that the United States would host the 2026 FIFA World Cup in a similarly open and festive manner, and that all eligible athletes, officials and fans from all countries around the world would be able to enter the United States without discrimination.”

Of course, a second Trump term would end in 2025, more than a year before the event, a fact the United Bid officials also have pointed out to voters. That has not seemed to matter, soccer officials from the United States, Mexico and Canada said Monday. What has eased the minds of some voters, the U.S. Soccer president Carlos Cordeiro said, is the mere existence of his letters — printed on crisp White House letterhead and marked with Mr. Trump’s distinctive, inch-high signature at the bottom in thick black pen strokes.

“You know, in this environment, he says that, in writing — it’s pretty powerful,” Mr. Cordeiro said.

In order to produce the letters from Mr. Trump, the White House began an interagency review to craft the language in them, according to a person familiar with the bid. Jared Kushner, Mr. Trump’s son-in-law and senior adviser, and his team also kept in touch with Canada and Mexico in what were sometimes the only harmonious interactions with the United States’ neighbors amid loud public clashes over trade and immigration.

And just as Morocco has enlisted current and former government ministers to take its 2026 message abroad, the United States government has at times played an even more active role: the National Security Council has been in touch with other countries whose votes could help put the United States over the top, the person familiar with the bid said, and Mr. Kushner leveraged his relationship with the Saudi royal family to get Riyadh to publicly announce its support for the North American effort.