Republican presidential candidate Herman Cain says his campaign problems stem from a political machine that's working relentlessly "to keep a businessman out of the White House."
That may be so. But Republican voters have denied the presidential nomination to businessmen for 70 years. And Americans haven't elected a president with Cain's background _ that is, no prior experience in elected office or war heroism _ since 1928.
It's certainly possible that voter attitudes are changing and Cain might surprise pundits. To do so, however, the Georgia businessman will have to overcome more than the sex harassment claims dogging his campaign. He will have to defy decades of political history.
So would four other GOP contenders, for similar reasons.
Not since Republicans chose corporate lawyer Wendell Willkie in 1940 has a major party nominated someone who had never held elected office or been a top military officer. Herbert Hoover, a mining engineer and Commerce secretary, was the last such person to be elected president, in 1928.
Three other current GOP candidates, with U.S. House backgrounds, also face tough historical odds. Americans haven't elected a president directly from the House since 1880, when James Garfield was the choice.
The past doesn't dictate the future, of course. Cain or Rep. Michele Bachmann of Minnesota might ride into history on a wave of intense public anger at government, which spawned the tea party movement in 2009. And not so long ago, the political establishment saw little chance of a black man being elected president.
But U.S. voters have followed some consistent patterns since the Great Depression. Fairly or not, these patterns help explain why many political strategists, analysts and journalists discounted Cain's chances from the start, along with those of Bachmann, who also thrived in Republican polls for a while.
And the patterns help explain why many political pros still feel the person best-positioned to challenge former Massachusetts Gov. Mitt Romney for the GOP nomination is Texas Gov. Rick Perry, even though he trails Cain and others in various polls.
If trends from the last seven or eight decades continue, only three current contenders have realistic shots at the nomination: Romney, Perry and former Utah Gov. Jon Huntsman.
Current and former governors head the list of four types of candidates who have been elected president in the 20 elections since Hoover's time. The other categories are senators; current or former vice presidents; and one military leader, Dwight Eisenhower.
All Democratic and Republican nominees since Willkie's time also came from those categories.
Garfield was the last sitting House member elected president, and few nominees have had House-only backgrounds. That doesn't bode well for Bachmann, Texas Rep. Ron Paul or former Speaker Newt Gingrich of Georgia. And Rick Santorum, the Pennsylvanian who lost his last Senate re-election bid in 2006, also doesn't fit the historic trends.
Governors have been especially successful in the past 100 years. Of the seven presidents who won multiple elections in that period, five started as governors: George W. Bush, Bill Clinton, Ronald Reagan, Franklin Roosevelt (elected four times) and Woodrow Wilson.
As a rule, some Americans will applaud Ross Perot's deficit-cutting plan, Steve Forbes' flat tax, or Cain's "9-9-9" tax plan. But they haven't elected a non-politician as president in generations
Romney's catchphrase is that he knows how to create jobs because of his private-sector experience. But voters seem to value his four years as governor, and his presidential try in 2007-08.
"There is a reason why non-politicians fail to win the White House," said political scientist John J. Pitney Jr. of Claremont McKenna College in California. "Running for office is like any other complex task: It takes practice to do it well, and one is likely to make big mistakes on the first try."
Most successful politicians make early mistakes in low-profile races, and are more polished when they reach a large audience, Pitney said.
"But nonprofessional presidential candidates make their mistakes on the national stage, where everybody notices," he said. He cited Perot's surprisingly strong showing as a third-party candidate in 1992 until he began making odd comments, such as claiming that Republican operatives wanted to disrupt his daughter's wedding.
Pitney said Cain "utterly botched the first rule of crisis management: Get your story straight before going public."
At a news conference Tuesday, Cain denied allegations from four women who say he sexually harassed them in the 1990s. As for their possible motivations, he said, "the machine to keep a businessman out of the White House is going to be relentless."
He vowed to keep campaigning.