It is all but impossible for you or for me to understand, let alone appreciate, how painfully public and publicly painful it must be for a losing major-party presidential nominee to bear the agony of losing the November general election.

The first line of your obituary has now been written. Your entire life is now unfairly defined by your public defeat.

The story has been told about a private conversation between former Vice President Walter “Fritz”​ Mondale, the 1984 Democratic presidential nominee, who had lost in a landslide to Republican President Ronald Reagan, and former Sen. George McGovern of South Dakota, the 1972 Democratic presidential nominee, who had lost in a landslide to Republican President Richard Nixon.

Hoping that McGovern might be able to offer some comfort, Mondale asked his fellow Democrat: “Tell me, George, when does it stop hurting?”

McGovern’s wistful answer: “I’ll let you know, Fritz.” He paused and then repeated, “I’ll let you know.”

Having worked, as a much younger man, full-time in three losing presidential campaigns, I learned firsthand the unmistakable signs of impending defeat the about-to-lose candidate and campaign face in October as the days grow short and the nights turn nippy.

Those few who survive in the brutal world of national political campaigns possess, I am sure, an extra olfactory nerve that enables them to detect, up to two time zones away, the scent of a loser who, after the polls close on Election Day, will be forced — graciously or gracelessly — to concede defeat to the world.

Enthusiastic crowds may continue to cheer the candidate, and the losing campaign will predictably attack the increasingly discouraging poll numbers (“Aren’t we Americans grateful that at Valley Forge, Gen. George Washington did not listen to some timid pollster who told him the Revolution was unwinnable?”

“Tell me, do you personally know anyone who was actually interviewed by any of these polls?”) But truth be told, by October, everything that happens in a campaign is a poll.

You, the presidential candidate, are in trouble when state party chairs, rather than ask you to headline the big rally at the state Capitol, invite your running mate, the less polarizing VP, to appear instead.

From fellow candidates in your own party, whose political fortunes could be adversely affected by your sinking popularity, you will hear truly imaginative excuses for why they are suddenly forced to bow out of publicly appearing next to you on the stage at major campaign events in their hometowns.

Examples: “Sorry, but I have a conflict. My favorite niece is, at that same time, graduating from scuba-diving school.” “Apologies, but I forgot our whole family’s appointment with the taxidermist to discuss how to best remember our beloved late canary.”

A losing presidential campaign, let me assure, does not necessarily build character. But a losing presidential campaign definitely reveals character.

After his last failed run for the White House, Sen. Ted Kennedy, D-Mass., responded with humor: “Frankly, I don’t mind not being president. I just mind that somebody else is.”

In his concession speech after his 2008 defeat, Sen. John McCain of Arizona was eloquent: “A little while ago, I had the honor of calling Sen. Barack Obama to congratulate him on being elected the next president of the country that we both love. … I wish Godspeed to the man who was my former opponent and will be my president.”

Now is crunch time, when nerves are raw and when the real possibility of defeat reveals a presidential candidate’s character. Keep your eyes open and pay attention.

Mark Shields is one of the most widely recognized political commentators in the United States. The former Washington Post editorial columnist appears regularly on CNN, on public television and on radio. Click here to contact him, or click here to read previous columns. The opinions expressed are his own.