Lee Atwater, who died too soon at the age of 41 from a brain tumor 20 years ago, had already managed the winning bare-knuckles 1988 presidential campaign of George H.W. Bush, who then appointed Atwater the chairman of the Republican National Committee.

Before he collapsed while speaking as party chairman at a March 5, 1990, Washington fundraising breakfast for then-Sen. Phil Gramm, R-Texas, Atwater, an expert judge of political horseflesh, had once confided that Gramm (whose presidential ambitions were already a semi-open secret) was the most disciplined candidate he had ever seen. If there were 10 calls for money or support on Gramm’s political must-do list for a Monday, Gramm, according to Atwater, would have all 10 completed and be looking for more calls to make — before 10 a.m. on Monday.

But, Atwater unequivocally added, Gramm (who would eventually run unsuccessfully for the White house in 1996) would never be elected to the nation’s highest office. Why? Because American voters, according to Atwater, insist upon a minimum level of likability in their presidents.

When one of his listeners asked him then to explain the victory of the painfully ill-at-ease Richard M. Nixon, who once privately told political aide John Sears that politics would be perfect — except for the (expletive deleted) people, Atwater said simply that in every century, there is one exception to that rule.

Atwater was almost certainly right. The vote for president is probably the most personal any of us casts. We are far more to evaluate and to vote for or against a member of Congress based upon whether we agree with the individual candidate’s public positions on the economy, education, health care and taxation than upon how we personally feel toward that congressional candidate.

That’s not the way it is in most voters’ presidential choices, where we have almost an information overload about the personality, childhood and family background of the presidential nominees — all the way to one of the candidate’s siblings publicly complaining to the nation how Mom always liked the candidate best.

What makes this anecdote theory relevant to 2012 is that the confidence of many of those seeking to re-elect President Barack Obama rely upon their conviction that the current field of Republican candidates (which, admittedly, has not generated enormous enthusiasm) contains no challenger personally as likeable or appealing to voters as the incumbent.

What that argument ignores is that whoever does emerge from the long, difficult primary slog to become the nominee will be given a large dose of benefit of the doubt by an electorate that respects anyone who can successfully navigate that political crucible and prevail. Understanding that one of the two major-party nominees almost inevitably be the next president, voters, other than blind partisans, assume there are some skills or strengths both the finalists.

So why did voters in 1968 bypass the more likable Hubert Humphrey for Nixon? Think about the national mood: There were a half-million U.S. troops fighting and dying in an unpopular war in Vietnam; U.S. cities, fraught with racial tension, erupted in riots; Martin Luther King Jr. and Robert F. Kennedy were assassinated; in short, the country was on the edge of a national nervous breakdown. Colorless but apparently competent, stability trumped likability.

If in the next election year, Americans are suffering through unemployment numbers and pain not felt since the Great Depression, Democrats should be nervous that voters in selecting a president could once again disregard Lee Atwater’s minimum level of likability rule.

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.