The letter in support of Mr. Trump, signed by 88 retired military figures, will come as the campaign prepares for a week focused on national security, with a forum hosted by NBC and MSNBC on Wednesday evening alongside Hillary Clinton, the Democratic presidential nominee.

“The 2016 election affords the American people an urgently needed opportunity to make a long overdue course correction in our national security posture and policy,” the letter states.

“As retired senior leaders of America’s military, we believe that such a change can only be made by someone who has not been deeply involved with, and substantially responsible for, the hollowing out of our military and the burgeoning threats facing our country around the world,” the generals and admirals write. “For this reason, we support Donald Trump’s candidacy to be our next commander in chief.”

Mr. Trump, the Republican nominee, is seeking to blunt an edge that Mrs. Clinton, a former secretary of state, has established either from retired military figures like Gen. John R. Allen, or from the tacit boost she could receive from Republican national security and foreign policy experts who have denounced Mr. Trump.