Last month, NBC Evening News embarked on what could have been a balanced effort to present national issues to millions of viewers. The concept: “My Big Idea,” an opportunity for each Democratic presidential candidate to offer one major policy proposal they would try to achieve if elected.

Too bad the end product only ended up showcasing the network’s progressive bias.

Some candidates offered vapid ideas, like Beto O’Rourke’s “returning power to the people” (whatever that means) or Congresswoman Tulsi Gabbard’s “peace and prosperity” (who’s against that?).

Others offered frighteningly grandiose plans to spend taxpayer money, like Sen. Elizabeth Warren’s demand for universal free college, universal child care and canceling student-loan debt.

To interview each candidate, NBC ­dispatched Harry Smith, a veteran correspondent and anchor. But in these individual segments, Smith and his producers simply let each candidate present a big idea virtually unfettered. The strongest colloquy featured Harry asking candidates: “How would you pay for this?” But in each instance, he simply accepted the candidates’ responses.

Perhaps Smith’s biggest pushback was to Sen. Bernie Sanders’ case for blowing up all private health care plans to make room for his one-size-fits-all, government-run health system. Smith commented to Sanders, “but the Republicans will call this socialism.” Sanders’ self-serving response was then permitted to be the last impression the segment left with viewers.

Covering Democratic candidates without affording coverage to opposing views makes such segments an enormous free megaphone for only one ideology and one party.

NBC’s website features the on-air interviews with 14 of the candidates so far, which usually run about 90 seconds each. But visitors can also access the “expanded” interviews with the candidates, which can run to more than nine minutes each. ­Already, that’s more than two hours of free advertising afforded to one side.

If NBC were to offer free, uncritical big-idea airtime to the remaining candidates, that would mean another two hours of unrestricted air and web time for liberal and radical nostrums.

None of the major networks offered similar opportunities to the GOP candidates in 2016 (although then-candidate Donald Trump’s unusual bid got a lot of airtime).

Each time the network offers the big idea of a liberal or radical candidate, there is an inherent imbalance in its coverage, unless it offers comparable time to conservative viewpoints. That could include covering an initiative from President Trump, perhaps explained by a cabinet secretary or a senior White House adviser.

Alternatively, NBC could provide time to policy experts from the conservative think tanks or from journals of opinion like The New Criterion, National Review, First Things or City Journal. To be fair, the network should allow each advocate similar amounts of air time and in the same format.

Rashida Jones, the senior vice president for specials at NBC News, explained the free rides given to the Democratic presidential aspirants, claiming that “our focus is on the candidates and how they plan to lead America.”

If NBC correspondents don’t challenge left-of-center views, as advanced by presidential candidates, it would only be fair to offer the conservatives equivalent, unimpeded time to express alternative big ideas, such as lower taxation, border security, school choice, free speech, the sanctity of life, military readiness, energy independence, streamlining bureaucracy and support for law enforcement.

This isn’t a case for a resurrected “fairness doctrine” or any other governmental intrusion. But to preserve their credibility across the nation’s political divides, NBC executives and journalists should voluntarily commit to fair and comparable coverage for opposing political philosophies, parties and candidates.

The hand of government isn’t always ­required for achieving the right result, ­especially when we citizens have the power of choice, right in that TV remote in our own hands.

Herbert Stupp was an NYC commissioner from 1994 to 2002, serving Mayor Rudy Giuliani’s administration. Early in his career, Stupp won an Emmy award for best editorial in the New York television market.