1. Bernie Sanders is now seen as at the far end of the political spectrum. He's climbing in the polls by inspiring the progressives in the Democratic Party (who are less than enthralled with the centrist history of Hillary Clinton). The fact that he has described himself as a "democratic socialist" -- using a word that's been used in this country for a century as a term of political abuse -- compounds Sander's vulnerability to being dismissed as the candidate of "the left."
A debate with Trump would offer Sanders an opportunity to show that his populist message can appeal to independents and people generally seen as on the right.
3. More important than what Sanders and Trump have in common is the profound difference between them. Demonstrating this difference publicly could be of vital importance, and a Sanders-Trump faceoff could accomplish this.
Both men are appealing to the dissatisfied. But while Trump's diagnosis and treatment are bogus, Sanders' speaks directly to what truly ails the United States. Trump is a demagogue using peripheral issues and phony arguments -- e.g. about immigration -- to distract people from what is really disempowering them and to their anger and frustration against scapegoats. Sanders, meanwhile, is calling attention to the real problems facing America's middle class and its citizenry in general-- the stacked deck that's leading to growing economic inequality and to the theft of our democracy by Big Money.
4. Bernie Sanders is the Democrat best able to expose the way Trump -- and the GOP generally -- are betraying their followers. Bernie has two relevant strengths here.
First, he is clearly authentic, which provides an important contrast to the Donald. Sanders ability to take on this task is not diminished by having the public free associate with his name the word "liar" (as it does, whether rightly or wrongly, with his main Democratic rival). The fact that throughout his career he has been outspoken and articulate on the issues of economic justice, and of people power versus Big Money power surely adds to his credibility.
And second, his considerable strength in argument and his temperament seem likely to make him especially effective in cutting through the blather and nonsense in which Trump specializes.
It might be objected that Trump -- now dominating the Republican field -- would have no incentive to share a stage with Sanders. But Trump might be the prisoner here of the chronically belligerent posture he has cultivated.
It would not be easy for Trump to walk away from a fight. Not the man who has gotten such mileage from showing himself prepared to take a swipe at anyone -- belittling John McCain's war record, going after Fox News and one of its signature reporters, throwing put-downs at Jeb Bush, Rick Perry, and Lindsey Graham.
Even if that doesn't succeed in getting Trump to join Sanders on a debate stage, it could get an equivalent of a debate going on in the media. (Can one imagine Trump not responding at all to such jabs?)
And such an exchange in the media would help to accomplish the same goals: get Bernie Sanders more attention in the mainstream media, give Sanders a chance to demonstrate appeal in the center and the right, and provide a large forum for the Democratic Party to expose the terrible disservice today's Republican Party is doing to its followers, and to the nation as a whole.