The special counsel, Robert S. Mueller III, has obtained a letter that President Trump and a top political aide drafted in the days before Mr. Trump fired the F.B.I. director, James B. Comey, which explains the president’s rationale for why he planned to dismiss the director.

The May letter had been met with opposition from Donald F. McGahn II, the White House counsel, who believed that some of its contents were problematic, according to interviews with a dozen administration officials and others briefed on the matter.

Mr. McGahn successfully blocked the president from sending Mr. Comey the letter, which Mr. Trump had composed with Stephen Miller, one of the president’s top political advisers. A different letter, written by the deputy attorney general, Rod J. Rosenstein, and focused on Mr. Comey’s handling of the investigation into Hillary Clinton’s private email server, was ultimately sent to the F.B.I. director on the day he was fired.

......... Mr. Miller drafted the letter at the urging of Mr. Trump during a weekend in May, when Mr. Trump and his team were at the president’s private golf club in Bedminster, N.J. During that same weekend, as Mr. Trump and a small group of aides were in Bedminster devising a rationale for Mr. Comey’s dismissal, Attorney General Jeff Sessions and Mr. Rosenstein were working on a parallel effort to fire Mr. Comey. ..........