You'd never think it from the present chaos in American presidential politics, but over most of the last century, relative stability has been the byword. Of the 12 elected presidents who ran for a second term in that period, eight were given it by the voters.
Only four -- William Howard Taft, Herbert Hoover, Jimmy Carter and the first George Bush -- failed to win re-election. Taft might have been re-elected too, had not former President Theodore Roosevelt, disappointed in him, jumped in as an independent Bull Moose candidate and delivered the election of 1912 to Democrat Woodrow Wilson.
Through all the ups and downs, though, there has been a general willingness to give the incumbent a second four years, regardless of party. The tradition of not changing horses in midstream, first argued most strenuously when Franklin Roosevelt sought his third of four terms in 1940, has pretty much endured in two-term incumbencies from Wilson through Barack Obama.
Even Richard Nixon won a second term in 1972, but of course he never finished it because the Watergate scandal forced him into the first presidential resignation in the nation's history.
Since the start of the two-term limit, passed by Congress in 1947 in the wake of FDR's fourth election and ratified in 1951, presidential elections have been encouraging many candidates to seek the Oval Office who have served as state governors, members of Congress or occasionally military leaders.
Nearly all have been political insiders of the two-party system, working their way up the ranks. In the Republican Party particularly, as I have noted before, loyalists have customarily waited their "turn" to be rewarded with the presidential nomination in terms of service in the party, as well as demonstrations of voter appeal.
But not this year, as is evident in the sudden emergence of party outsiders to the front ranks in the public opinion polls. Not only the Donald Trump phenomenon, but also the early public support for Ben Carson, the retired neurosurgeon of soft-spoken demeanor and conservatism, and for former businesswoman Carly Fiorina have shaken up the old order, denouncing political business-as-usual as practiced in Washington and other halls of electoral power.
For once, it almost seems that the absence of conventional political credentials is a selling point to the public, especially in the case of Mr. Trump, whose bombast and even personal crudity is elevating him to the stature of a pied piper of popular anger at the "ruling class."
In the Democratic Party as well, the fervid and energetic harangues of declared socialist Sen. Bernie Sanders of Vermont, an independent seeking the Democratic nomination, has tapped into the same public discontent with the regular political order. Taking as his own the same fight for the middle class and for income equality that front-runner Hillary Clinton has embraced, Mr. Sanders has sent shock waves through her campaign, which is already troubled by allegations of impropriety in the management of her old State Department correspondence and papers.
On Wednesday, after finally uttering a flat apology for her use of the private email server in the hope of quieting that furor, Ms. Clinton pivoted to a strong foreign policy posture apparently designed to counter Mr. Sanders. While supporting President Obama on the nuclear deal with Iran, she also cited some notable differences with her old boss, striving to reassure those in the party who have been critical of his less aggressive attitudes toward the Russian adventurism in Crimea and Ukraine, the Syrian rebellion and the Islamic State emergence.
All this sudden interest and attention in both major parties in the actions and challenges of figures outside the regular political order is upsetting tradition and requiring a deeper assessment of today's more angry and less orthodox electorate.
Conventional wisdom, as well as common sense, continues hold within the political establishment that the American electorate will not be so foolhardy in the end to nominate or elect a self-aggrandizing demagogue like Donald Trump. But until a Republican alternative emerges to neutralize or counter the public anger that feeds him, it's going to keep a circus carnival going on out there.
Jules Witcover is a syndicated columnist and former long-time writer for The Baltimore Sun. His latest book is "The American Vice Presidency: From Irrelevance to Power" (Smithsonian Books). His email is email@example.com.