When it comes to businesses, the impacts of monopolies are well documented. Lacking any of the benefits that flow from competition, customers can count on paying higher prices for the privilege of getting inferior service.

Alas, the same is true of government. New Yorkers know too well that a political monopoly — one-party rule — yields some of America’s highest taxes while services and results are mediocre at best. Think public education and mass transit.

As Election Day approaches, those facts are worth keeping in mind — along with another unhappy distinction. Two of New York state’s top Democrats, Gov. Andrew Cuomo and Sen. Kirsten Gillibrand, are seeking re-election even as they dream of becoming their party’s nominee in the 2020 presidential race.

Leave aside whether their dreams are foolish fantasies. For voters and taxpayers, the conflicts of interest between the jobs they want to keep and their national ambitions are compounding bad government.

Consider, for example, what will happen if they are re-elected. Even before they take the oath, they will be plotting a national schedule and making fund-raising calls as the 2020 preliminary season gets underway.

As soon as they are sworn in to new terms in January, they’ll start traveling around the country to drum up support for the nomination. If they get any traction, they will travel more and focus more on national issues and treat their day jobs as little more than taxpayer-financed stepping stones.

Already, their conflicting loyalties are depriving New Yorkers of honest services. Cuomo has tried to turn the Empire State into a lab experiment for policies he believes will make him look good to national Dems.

Many of his press releases tout a “first-in-the-nation” claim, as if the states are in a race for bragging rights, when that claim is in his interest only. New Yorkers get no advantage from being first with policies that waste money, as nearly all of his economic-development initiatives have done.

Recall the tens of millions he spent advertising “the new New York.” The TV campaign did more for Cuomo’s national name recognition than it did for the upstate economy, unless you count the boosted bank accounts of defense lawyers growing out of the resulting corruption.

Similarly, Gillibrand is completely oblivious to the biggest problems facing New York. With the region’s mass-transit system teetering, she is missing in action when it comes to solutions, despite being a member of the Senate’s Environment and Public Works Committee, which helps set national transportation funding.

Although the metropolitan area has 40 percent of the nation’s mass-transit ridership, it gets only about 16 percent of federal mass-transit funds, according to Gillibrand’s GOP/Conservative opponent, Chele Chiavacci Farley.

Farley also cites the federal tax bill as another area where Gillibrand did nothing to help the people who hired her. Although changes in rates and other factors will mean a tax cut for some 80 percent of New Yorkers, limits of $10,000 on deductions of state and local taxes could mean higher federal taxes for some upper-income New Yorkers. Yet Gillibrand voted no on the tax cuts without engaging the White House. Her refusal to negotiate, Farley says, meant Gillibrand put party loyalty and national ambitions ahead of the taxpayers she ostensibly represents.

“Senate Republicans admit there was nobody at the table to represent the high-tax states,” Farley told The Post. “There are no Republicans in the Senate from California, New Jersey, New York, Connecticut — not a one.”

Meanwhile, Cuomo’s Republican/conservative opponent, Marc Molinaro, is focused on Cuomo’s performance in Albany and argues that giving the governor a third term would be a disaster.

“We need an absolute cleansing of state government,” he said in a recent meeting with The Post’s editorial board. Molinaro, the Dutchess County executive, says that if Washington is a swamp, “Albany is a cesspool.”

He cites what he calls the “death spiral” of the MTA, which Cuomo controls, and vows to block any fare hikes or congestion pricing until the agency reforms its spending and gets control of the mammoth waste on big projects.

Anyone in doubt about the damage of pols who have lost interest in their jobs need only look at Bill de Blasio. Mayor Putz is giving us a sneak peek by spending all his time pursuing a spot on the national stage instead of running the city.

Molinaro and Farley offer fresh thinking on old problems, but are fighting uphill battles in deep-blue New York. Gov. George Pataki was the last Republican to win statewide office in 2002.

That was a lifetime ago in politics, and the sorry results of the Dems’ rule are typical of what monopolies generally produce. If New Yorkers want better service for less money, they need to vote for competition and new ideas.

No horse sense

It is impossible to defend or adequately explain President Trump at times. “Horseface” may be the ultimate case in point.

The president tweeted that word to refer to Stormy Daniels after a judge threw out her defamation case and awarded him damages. She responded by calling him “tiny,” which makes them even.

Except he’s the president of the United States. Yet after touring the horrific storm devastation in Florida while trying to sort out the facts around the murder of a Saudi journalist in Turkey and trying to keep his party’s narrow majorities in Congress, among a thousand other things, he celebrates his court victory by taking a dive into the mud with a porn star.

Why?

It Warrents the shame

Most Americans were not talking much anymore about Elizabeth Warren’s old claim that she has Native American ancestry. Although Trump was still calling her Pocahontas, even the put-down had lost its novelty.

Until Warren’s giant blunder. By taking a DNA test and declaring it validated her claim despite showing she probably had one Indian ancestor as many as 10 generations ago, Warren revived the issue and even angered leaders of the Cherokee Nation. They shot down her assertions of being a descendant, saying it was “inappropriate and wrong” to use DNA tests to cite tribal heritage.

The best summation of the odd debacle came from Megyn Kelly at NBC. Warren, she said, “scored a goal against herself.”

McCrayzy expense

The most laughable of City Hall’s defenses for first lady’s Chirlane McCray’s 30 out-of-state trips, at taxpayer expense, was the assertion that a professional photographer needed to travel with her sometimes. A smartphone and a selfie would be cheaper.