Is it unthinkable that Bernie Sanders and Donald Trump could win their parties' nominations for president? In writing about the possibility of thermonuclear war, the iconic Cold War strategist Herman Kahn challenged his readers to "think the unthinkable." In writing about politics, here is another "unthinkable" proposition to ponder.

Suppose Democrats reject Hillary Clinton (or Hillary withdraws over the email or other exploding political time bombs). And suppose celebrity status becomes the Republican metric for electability.

Further suppose that a majority of the American electorate, outraged by the failed politics and broken government in Washington, turn to non-traditional candidates. Is a Sanders versus Trump choice for president "unthinkable?"

Common sense argues not only yes but hell yes! Wiser heads in the GOP would never tolerate a Trump-led ticket even though the former real estate mogul and television star threatens to split the Republican Party if he is not treated "with respect."

If Hillary falters, Vice President Joe Biden could enter the lists possibly to serve as a one-term president, while picking a particularly attractive number two -- perhaps one of the two Virginia senators or a well regarded governor from either party -- who could fleet up in four years time to the top spot.

But, for a moment, suspend reality and think the unthinkable. Could a Sanders-Trump run-off actually occur and, aside from guaranteeing a radioactive and spectacular media circus, what would that race mean for America? That specter should keep Americans awake at night.

Clearly, voter animosity and disenfranchisement with Washington would be the forcing function. For Mr. Trump, that means his current lead in the polls is not a ceiling. The contagion of his loyalists would spread to other Republicans and, more importantly, to independents.

Trump's celebrity status and Pied Piper ability to attract millions of viewers to watch the first debate are his major (and probably only) strengths. If Trump is serious about winning and reaches deeply into what he brags are his well-lined pockets to invest many tens or hundreds of millions of dollars in winning the nomination, he will overtake and outdistance even the well funded Jeb Bush. Sadly, today, money dominates politics, often determining winners and losers.

For the Dems, Sanders is riding high. If Hillary is not mortally wounded and does not withdraw or concede, the same disgust with Washington could propel Sanders to the nomination. The other Democratic contenders are so far behind as to be irrelevant.

If Hillary does drop out, a Biden or other late entry would challenge a Sanders nomination. Age would not be an issue as both Biden and Sanders are in their seventies. Despite his many qualities and experience, Biden is gaffe prone. And a late entry into the race could be too late given Sanders' uncanny ability to attract tens of thousands to his rallies and speeches.

What might a Sanders or Trump presidency look like? Even though it is early in the race, that frightening question is unanswerable. Sanders and Trump only offer vague promises about fixing broken government that ignore reality and lack substance and credibility. The respective websites of both candidates are useless. The Trump website is a paid infomercial selling the Donald without a singe policy prescription.

Sanders' website is aggressively progressive, socialist and naïve if not scary. The world is literally ablaze. But nary a word is mentioned about foreign policy, national security and dangers to America other than those posed by Wall Street and the billionaire class.

The eight issues listed on Sanders' site are entirely domestic, ranging from income and wealth inequality to getting money out of politics to real family values, climate change and environment and, of course, reforming Wall Street. The policy prescriptions are a mélange regurgitating the senator's largely unsuccessful prior pronouncements to deal with these issues. And his promise of a trillion dollar government program to modernize the nation's failing infrastructure ignores who would pay for it and how it would be funded.

Trump boasts that he will make America strong again; fence off Mexico with Mexican money; rebuild the military; and create millions of jobs. How this is to be done remains a work very much in progress and thus "missions unaccomplished." At some time, perhaps Trump, the showman and prestidigitator, will provide substance to these grandiose wishes. We will see.

Returning to Herman Kahn, a thermonuclear war thankfully was never fought. And, as remote is the specter of a Sanders versus Trump race. Unthinkable -- one hopes.

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Harlan Ullman is UPI's Arnaud de Borchgrave Distinguished Columnist; Chairman of the Killowen Group that advises leaders of government and business; and Senior Advisor at both Washington D.C.'s Atlantic Council and Business Executives for National Security. His latest book is "A Handful of Bullets: How the Murder of Archduke Franz Ferdinand Still Menaces the Peace."