Walter Mondale created the modern vice presidency out of a position that was an afterthought at the Constitutional Convention. He served as a full partner, and with the full support, of President Jimmy Carter. Every administration since has followed the model.

Mondale was a great public servant and a decent man, and his death comes at a time when his progressive politics have gotten a second breath in the Democratic Party after a long dormant period. For four years I saw him up close. He confided in me, sharing not only his respect for Mr. Carter but his frustrations, which he never aired publicly.

The Founding Fathers hardly focused on the vice presidency. The Constitution makes him—now her—a member of the executive branch and first in line to succeed the president—but also a member of the legislative branch, as president of the Senate eligible to vote only to break a tie.

Originally the vice president was the man who finished second in the Electoral College, but that proved unworkable when President John Adams’s opponent, Thomas Jefferson, became his understudy. After the 12th Amendment’s ratification in 1804, the vice presidential candidate ran on the same ticket, and his principal function was to deliver votes through regional or ideological balance. Franklin D. Roosevelt’s first vice president, John Nance Garner, is said to have likened the office to bucket of warm spit.

In 1945 FDR concealed the atomic-bomb project from Vice President Harry S. Truman, who learned of it only on Roosevelt’s death. When President Dwight D. Eisenhower was asked what major responsibilities he’d given Vice President Richard Nixon, Ike sniffed that he would need a week to try to recall one. President John F. Kennedy deliberately marginalized Vice President Lyndon B. Johnson, formerly a powerful Senate majority leader; it was a deep wound LBJ never forgot.

Yet I know firsthand that LBJ later treated his vice president, Hubert Humphrey —mentor to Mondale, his young Senate successor—in the same cavalier fashion. I was a young staff aide, and my small office in the White House complex was next door to the vice president’s. Humphrey’s staff regularly complained about their boss’s humiliations. He had no meaningful role and often sat, stewing, as he waited half an hour to meet with a White House staffer.

A principal theme of Mr. Carter’s race for the 1976 Democratic nomination was his promise to restore trust in government after Watergate. Among his goals was raising respect for the vice presidency, tarnished by Spiro Agnew’s 1973 resignation. Before Mondale was asked to be considered as Mr. Carter’s running mate, he met with Humphrey, who had returned to the Senate in 1971. Mondale expected his colleague would advise him to refuse any offer. Instead, Humphrey urged him to accept. Despite the indignities, Humphrey said, he had “more opportunity to affect public policy down there in one day than I have up here in a decade.”