Few top officials spent more time behind closed doors in the White House with President Donald J. Trump than John F. Kelly, the former Marine general who was his longest-serving chief of staff.

With Election Day looming, Mr. Kelly — deeply bothered by Mr. Trump’s recent comments about employing the military against his domestic opponents — agreed to three on-the-record, recorded discussions with a reporter for The New York Times about the former president, providing some of his most wide-ranging comments yet about Mr. Trump’s fitness and character.

Mr. Kelly was homeland security secretary under Mr. Trump before moving to the White House in July 2017. He worked to carry out Mr. Trump’s agenda for nearly a year and a half. It was a tumultuous period in which he drew internal criticism over his own performance and grew disenchanted and distressed by conduct on the part of the president that he considered at times to be inappropriate and reflecting no understanding of the Constitution.

In the interviews, Mr. Kelly expanded on his previously expressed concerns and stressed that voters, in his view, should consider fitness and character when selecting a president, even more than a candidate’s stances on the issues.

“In many cases, I would agree with some of his policies,” he said, stressing that as a former military officer he was not endorsing any candidate. “But again, it’s a very dangerous thing to have the wrong person elected to high office.”