DENVER – There’s a saying in the news business that when something happens once, it’s an anomaly.

If it happens twice, it’s a coincidence.

Three times? Well, that’s a trend.

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We’ve spotted a trend – and it isn’t a good one.

On Nov. 5, voters in Denver, Boston and New York City will cast ballots in minor, off-year elections that will go a long way toward determining the future of charter schools in each of those three major U.S. cities.

As a result of these “high stakes, low turnout” contests, tens of thousands of families could see their access to the alternative public schools – which serve as academic lifelines for many children – greatly diminished if teacher union-backed mayoral and school board candidates muscle their way to victory next month.

It’s a dream scenario for the well-organized labor unions, which can conceivably roll back years of school choice progress in those cities while most taxpayers aren’t even paying attention.

Charter schools face their most imminent threat in New York City, where Democratic mayoral candidate Bill de Blasio leads Republican Joe Lhota by some 40 points in the polls. De Blasio has publicly said the city doesn’t need any more charter schools and that existing charters housed in public school buildings should pay rent to their government-school landlords.

Charter parents and supporters are so worried by de Blasio’s “freeze ‘em out, starve ‘em out” strategy that 20,000 of them marched on New York City Hall last week as a kind of warning – or plea.

Bostonians, meanwhile, will choose a new mayor to replace the incumbent, who’s retiring after 20 years in office. Analysts say Beantown’s next mayor will play a significant role in whether or not state lawmakers agree to lift the city’s current charter school cap. Without an increase, some 16,000 children will continue to languish on a charter school waiting list.

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Both of these mayoral races have received significant media coverage. That’s not the case for the Denver school board race, which is being billed as “the most important political race you’re not paying attention to.”

Denver’s impressive reforms hang in the balance

Next month, voters will elect four individuals to the Denver school board. Reformers currently hold a slim 4-3 majority on the board, but that could change very easily. Two of the open seats are currently held by reform-minded board members, and two by teacher union-friendly members.

The direction of Denver Public Schools is truly in a jump ball situation.

According to The Denver Post, “a shift” in the board’s composition could cost Superintendent Tom Boasberg his job. That would be devastating to the city’s school reform effort, which has been described as a “model” for other large urban school districts.

During Boasberg’s five-year tenure as Denver’s school leader, the district has more than doubled the number of charter schools (from 19 to 41) and has added 15 innovation schools, which offer students a unique and challenging curriculum.

He has also pulled the plug on 16 failing schools, opened 13 traditional campuses, and implemented the district’s new teacher evaluation system that links teachers’ job reviews to their students’ academic growth.

Between Boasberg and his predecessor, the 84,000-pupil district “has increased its graduation rate, cut the dropout rate in half and experienced student academic performance growth,” the Post reports.

Many of Boasberg’s policies will come to a grinding halt if the Denver Classroom Teachers Association – the local teachers union – succeeds in getting its favored candidates elected to the school board.

Union-backed candidates “want a moratorium on charter schools in the district and a greater focus placed on improving existing campuses,” the Post reports. “They say they are not against evaluating teacher performance, but want those evaluations to be accompanied by more training and a holistic approach that focuses less on standardized test scores.”

In other words, they want to go back to the bad old days, when children were stuck in failing schools with no alternatives, and unskilled teachers were allowed to maintain their jobs.

The stakes couldn’t be much higher, which is why both sides of the reform debate – both locally and nationally – are expected to spend heavily on advertising and get-out-the-vote efforts over the next three weeks.

That’ll likely be the case in Boston, and even in New York City, where charter leaders “are seeking donations in case they are forced to pay rent,” the New York Times reports.

And that brings us back to this troubling election trend.

Regardless of how one feels about charter schools and education reform, Americans should be able to agree that these are important issues deserving of a serious and vigorous public debate.

But allowing these critical issues to be decided in off-year, second-rate political contests is unbecoming of the world’s greatest representative democracy.