Based on favorable comments he's made in the past on Cuba, Nicaragua and even the Soviet Union, Democratic front-runner Sen. Bernie Sanders may lack the legitimacy required to confront autocratic nations and leaders.

Not familiar with this history? You will be, if Sanders is the nominee. In fact, some rivals are already alarmed over potential Trump campaign commercials featuring footage of Sanders' comments on his travels.

The Democratic Party should be concerned about the potential impact on the election. More profoundly, Americans should consider how Sanders would govern. The Vermont senator showed naiveté, at best, about the nature of repressive regimes. Sure, he offered some muted criticism. But it was couched in compliments that overlooked the oppression that was the defining dynamic in each country.

After his Soviet visit in 1988, for example, Sanders didn't focus on the Orwellian Soviet system that led to deaths or dead-end lives of millions in multiple countries. Instead, he noted the country's chandelier-laden subway stations. "They put a lot of money into culture, they want people to enjoy it, and they deserve credit for that," Sanders said. (Never mind that Aleksander Solzhenitsyn and others were put in gulags for cultural expression.)

Cuba's revolutionary spirit was "far deeper and more profound than I understood it to be," Sanders said after a 1989 visit to that nation. This wasn't youthful naiveté — Cuba's revolution, like Sanders himself, was middle-aged when he made such comments. And just last Sunday on "60 Minutes" Sanders praised Fidel Castro's "massive literacy program" without voicing concern about the regime's monitoring of what Cubans read — and wrote.

Sanders also said in 1985 that Sandinista leaders impressed him with "their intelligence and their sincerity." The insincere leader of that thuggish government, Daniel Ortega, leads Nicaragua once again in what the State Department's 2018 Human Rights Report called a "highly centralized, authoritarian political system" that's engaged in egregious human rights abuses, including a policy of "exile, jail or death for anyone perceived as opposition."

There were similarly grim reports on Cuba — "unlawful and arbitrary killings by police; torture of political dissidents, detainees and prisoners by security forces; harsh and life-threatening prison conditions; arbitrary arrest and detention; holding of political prisoners; and arbitrary or unlawful interference with privacy." And beyond oppressing its own citizens, Russia has invaded Ukraine and attacked Western elections, according to the FBI, "to get Americans to tear themselves apart." That's not just an issue from the past, but one with bearing on the future.

Sanders has recanted some of his previous equivocation on repressive regimes, including the heinous failures of Venezuela's socialist leader, Nicolas Maduro. But his history is deeply troubling and could compromise his ability as president to press for free elections, and free people, everywhere.