By Cokie Roberts and Steven V. Roberts

Bernie Sanders was speaking recently to a gathering of like-minded liberals in Chicago.

“I am often asked by the media and others, ‘How did it come about that Donald Trump, the most unpopular presidential candidate in the modern history of our country, won the election?’” he said. “My answer is that Trump didn’t win the election — the Democratic Party lost the election.”

True enough. But here’s what Sanders and his supporters refuse to admit: Bernie himself was a major reason why the Democrats and Hillary Clinton failed.

Of course Clinton was a poor candidate, with visible scars and an invisible message, who made many unforced errors. It was crazy for a woman who intended to run for president as a defender of middle-class families to take gobs of money from Wall Street.

But Sanders was an irresponsible candidate who advanced an impossible set of proposals that bore little relation to reality. It’s both careless and cynical to yell “free” at every opportunity with absolutely no notion of how to pay for all those goodies. (The Wall Street Journal estimated Sanders’ proposals would cost $18 trillion.)

The result was to excite Sanders’ base, especially young and inexperienced voters, and deepen their disillusionment with Clinton, who was burdened with a sense of practicality and responsibility. Sanders aggravated their disdain by indulging his ego, staying in the race long after the outcome was clear and implying that a rigged system designed by establishment mandarins had cheated him out of the nomination. No wonder so many Berniecrats voted for third-party candidates or simply stayed home.

This would all be ancient history, except that Sanders is still singing the same old song. As he said in Chicago, “The current model and the current strategy of the Democratic Party is an absolute failure.”

But why should Democrats listen to him? After all, he is not a Democrat. He’s a self-proclaimed socialist who ran for the Senate as an independent and still identifies himself that way.

Moreover, while the system is far more polarized than in the past, the rules of arithmetic have not been repealed. The Berniecrat Creed — that the Democrats must shift sharply to the left in order to succeed — is a recipe for disaster.

In the last election, only 26 percent of voters identified as liberals, while 35 percent called themselves conservative and 39 percent moderates. This is not a leftist country, and every time the Democrats have followed that fantasy, they have been crushed.

Take 1972, when George McGovern rode a Sanders-like wave of anti-war emotion to the Democratic nomination. He then won 37.5 percent of the vote and 17 electoral votes against Richard Nixon — a man totally lacking in warmth, charm or scruples. Walter Mondale raised the liberal banner against Ronald Reagan in 1984 and did even worse: 13 electoral votes. Michael Dukakis did slightly better four years later, but he still managed to lose to George H.W. Bush, a mediocre candidate whose re-election was later thwarted by a more centrist Democrat, Bill Clinton.

There are important lessons going forward here for the Democratic Party. As The New York Times reports, the party is “facing a widening breach” as it prepares for next year’s elections, a “growing tension between the party’s ascendant militant wing and Democrats competing in conservative-leaning territory.”

A strategy that focuses on energizing the Democratic faithful on the west sides of Los Angeles and Manhattan is not going to retake Congress in 2018 or challenge Trump’s re-election in 2020.

This is all playing out in suburban Atlanta, where Democrats think they have a real chance to capture a House seat vacated by former Rep. Tom Price when he joined Trump’s Cabinet. The Democratic candidate in the June 20 contest, Jon Ossoff, is reflecting the district and running as a card-carrying moderate, far from the Sanders mold.

His “message to Washington,” he told the Times, is about “decency and respect and unity, rather than division.” Stephanie Runyan, one of Ossoff’s precinct captains, added, “A lot of us are not true-blue liberals.”

Rep. Emanuel Cleaver, a Missouri Democrat and former chairman of the Congressional Black Caucus, echoed that argument on a national level. “We are going to lose every possible winnable seat, in a year where there are many winnable seats, if we come across as inflexible left-wingers,” he told the Times. “I respect Bernie — I just don’t think we can become the party of Bernie.”

No, they can’t. Not if they want to win.

Steve and Cokie Roberts can be contacted by email at stevecokie@gmail.com.