A cheering audience is a political aphrodisiac. Candidates for president, moved by crowds' affection, become convinced that they are right, their opponents wrong. The pout of political pride makes backing off difficult later on.

Every campaign, no matter how humble and reasonable the candidate at the beginning, encourages a growing conviction of unique importance. This year, Donald Trump is the leading example, but self-importance is not a partisan condition. Sen. Bernie Sanders is justifiably gratified by his leap from obscurity to a formidable string of primary victories and special success in capturing the hopes of millions, particularly the young and idealistic.

But Sanders' criticism of Hillary Clinton — constant and repetitious — has become increasingly bitter. That may be productive now, but those words do not disappear when the convention makes a choice and it is not him. They become weapons for the political enemy. In the mouth of Trump, particularly, they would become bludgeon and meat ax.

Sanders should calm down, save the vitriol and think of the consequences of not doing so.

I would tell him to tame his rhetoric, put snide aside and, most of all, prepare himself to endorse Clinton immediately at the convention — and to do it without equivocation, with more enthusiasm than he may now think possible.

My advice grows from my experience nearly half a century ago, in 1968. It was a difficult year for the country and particularly for candidates for national office. The Democratic convention was mean-spirited inside and brutally chaotic outside.

Challenged primarily on his Vietnam policies by Sens. Eugene McCarthy and Robert Kennedy, President Lyndon Johnson had announced he would not run again. After a bit, his vice president, Hubert Humphrey, announced that he would run, although he waited until it was too late to enter any primaries, avoiding the chance of a loss.

After Kennedy was assassinated, Sen. George McGovern took up his anti-Vietnam crusade, and he and McCarthy — like Bernie Sanders today — aroused huge, enthusiastic crowds, including many young people.

McCarthy patronized Humphrey as weak, sometimes ridiculed him, made him damaged goods for the general election. But Humphrey ultimately accumulated a huge lead in delegate votes.

McCarthy, McGovern and Kennedy had provided focus for the antiwar movement and for those who, for whatever reason, were fed up with Johnson and Humphrey. It took courage on their parts. And they had served the country well.

Once Humphrey was nominated despite their policy differences, McGovern immediately announced his support for Humphrey. McCarthy was silent. Almost immediately after the convention, McCarthy took off for a vacation in Europe. When he returned several weeks later, he remained mute and elusive. When he did finally speak, just a week before Election Day, he said only that he would vote for Humphrey. Nothing more.

Ultimately, Humphrey lost the election by less than a percentage point. States where McCarthy was immensely popular might have been won with his support and would have provided the electoral votes needed for election.

Sixty-one million votes were cast. In Missouri, Nixon won by fewer than 25,000 votes. McCarthy might well have brought that many to the polls. In three states — Illinois, Missouri, and Ohio — if 125,000 votes had been switched, America would have not suffered a Richard Nixon presidency. Humphrey would have won.

Why does this matter almost 50 years later? It matters because Sanders must soon decide whether to be a McCarthy or a McGovern.

Toning down his rhetoric now would be a good step in the right direction.

Norman Sherman worked for both Eugene McCarthy and Hubert Humphrey. He was Humphrey's press secretary during his vice presidential years. This column is a revised excerpt from his memoir, "From Nowhere to Somewhere."