The two men have been very intimate and reliable friends throughout their lives.

For most of their lives, the two men had shared confidences, swapped advice and supported each other’s ambitions. The dozens of letters that Justice Blackmun saved document a relationship of sometimes startling intimacy.

Mr. Bush, the son of a president and the grandson of a senator, castigated Mr. Kerry as a Washington insider who has been in Congress so long that he has taken both sides on just about every issue.