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The national field director for Donald J. Trump’s campaign — who is a loyalist to Corey Lewandowski, the embattled campaign manager — resigned in a letter to Mr. Trump on Monday afternoon.

“I want to express my deepest gratitude for the opportunity to serve you and your campaign over the past 7 months,” Stuart Jolly, the director, wrote in his resignation letter. “The journey has been extraordinary and many experiences on this journey will never be forgotten.”

Mr. Jolly, a retired Army lieutenant colonel, was especially loyal to Mr. Lewandowski, whom he had worked with previously at Americans For Prosperity, the free enterprise advocacy group associated with the billionaire brothers Charles G. and David H. Koch.

In the letter, Mr. Jolly stressed to Mr. Trump that his departure “has nothing to do with you or Corey’s staff, because I have never worked with a finer group of people.” The letter, and a major staff meeting on Saturday, were reported by the website Politico.

“The friendships I have made through your campaign will last a lifetime; my hope is that ours will as well,” he wrote, calling Mr. Lewandowski “one of my best friends” and offering his availability in the future “if you ever need me for any reason.”

But the resignation was seen as a sign of distress among those loyal to Mr. Lewandowski about recent changes in the campaign that have curtailed his influence after a string of losses in states like Wisconsin and at state conventions to select delegates.

Mr. Jolly, whose first national campaign experience came this cycle under Mr. Trump and who was deployed to help the campaign in New Hampshire primary after a disappointing second-place finish in Iowa, also reminisced about Mr. Trump’s early victories, which helped transform the Manhattan businessman from something of a political joke into a serious contender.

“The primary wins in New Hampshire & South Carolina were magical and I will never forget them,” Mr. Jolly wrote. “The other 20 wins were nice, but nothing beats those first two victories. Victory has no substitute!”

“I will never forget your encouragement and loyalty,” Mr. Jolly added, before offering Mr. Trump some final advice — to continue to listen to those who had helped propel him to victory.

Aides to Mr. Trump did not respond to requests for comment.

The comments in the letter about Mr. Lewandowski’s successes came on the heels of a senior staff meeting on Saturday, during which it was made clear that Paul Manafort, Mr. Trump’s new senior adviser, was largely steering the ship, according to people briefed on the meeting. Mr. Manafort introduced a new lawyer on the team, William McGinley, who has dealt in the past with delegate battles.

Mr. Trump’s son-in-law, Jared Kushner, joined the meeting for part of it, and Ed Brookover, who had been serving as the liaison to the Republican National Committee, also attended. Other attendees were Mr. Lewandowski and his deputy, Michael Glassner, as well as Rick Reed, who films the campaign’s ads.

Mr. Manafort laid out a plan for the coming weeks, one with tighter messaging and a broadened communications staff, which could increase as soon as this week, according to those briefed on the meeting. Midway through the meeting, Mr. Glassner gave what was described as a defense of the work that had come before the new arrivals, pointing out that the old team had won more than 20 states.

Mr. Manafort last week hired Rick Wiley, a former party committee political director, as the campaign’s national political director. He was said to have been recruited by Mr. Lewandowski, but now reports to Mr. Manafort. That put Mr. Jolly beneath Mr. Wiley in the chain of command.