WASHINGTON — In the spring of 1988, Justice Thurgood Marshall assigned a clerk, Elena Kagan, to write a first draft of his opinion in a case considering whether a school district could charge a poor family for busing a child to the nearest school, which was 16 miles away.

A majority on the Supreme Court ruled that the busing fee was constitutional. Justice Marshall, who was 80, was incensed and wanted a fiery dissent. But the 28-year-old Ms. Kagan, now a Supreme Court nominee, thought her boss’s legal analysis was wrong.

Ms. Kagan, recalling the incident in a 1993 tribute after his death, wrote that after she told him that “it would be difficult to find in favor of the child” under legal doctrine, he called her a “knucklehead.” He “returned to me successive drafts of the dissenting opinion for failing to express — or for failing to express in a properly pungent tone — his understanding of the case,” she wrote.

Because Ms. Kagan has never been a judge and has produced only a handful of scholarly writings, clues to her philosophy are rare. In that vacuum, liberals and conservatives alike are attributing special significance to her clerkship year with Justice Marshall, who led the civil rights movement’s legal efforts to dismantle segregation before becoming a particularly liberal Supreme Court justice.