In 1846, New Yorkers approved a 20,000-word state constitution that requires the electorate, every 20 years, to answer this question: “Shall there be a convention to revise the constitution and amend the same?”

Since that time, voters have had the good sense on a majority of occasions to reject that proposition. In the past 150 years, they approved only five conventions and overwhelmingly rejected the proposed revisions four of those five times. Hopefully, they will do the same when they go to the polls this November.

Holding a convention is a bad idea, because it could easily become a field day for New York’s special-interest groups, which will dominate the machinery to elect delegates. If history is a guide, it is fair to assume a constitutional convention will not be dominated by elected delegates who are civic-minded citizens, frustrated taxpayers or constitutional scholars. At the last gathering — held in 1967 — 91 percent of the delegates were local pols or union officials.

And the usual political suspects were convention officers: Assembly Speaker Anthony Travia served as the convention’s president; New York City Mayor Robert Wagner as first vice president.

Other key officers included the Legislature’s majority and minority leaders.

Fortunately, 72 percent of the voters rejected the package of constitutional changes proposed at that 1967 gathering.

This year, interestingly enough, public-employee unions and the New York Conservative Party are collaborating to defeat the November referendum. The unions oppose a convention for self-serving reasons: They fear the loss of constitutionally protected government-employee pension guarantees.

The unions are intent on preserving about 50 words of the now 50,000-word constitution.

There’s Article V Section 7, which reads: “Membership in any [public] pension or retirement system . . . shall be a contractual relationship, the benefits of which shall not be diminished or impaired.”

Then there’s Article XVI Section 5, which states: “All salaries, wages and other compensation, except pensions, paid to officers and employees of the state and its subdivision shall be subject to taxation.” (Italics added.) Most taxpayers likely don’t know that New York’s civil servants pay no state or local income taxes or Social Security on their guaranteed-for-life pensions.

Conservatives and others, on the other hand, have a broader view. They fear an out-of-control convention could prescribe:

Elimination of the requirement for voter approval of state general obligation debt.

Elimination of the Executive Budget Amendment, which gives the governor the responsibility for drafting the budget.

Elimination of the governor’s line-item veto.

Elimination of restrictions on indebtedness of local governments and school districts.

Elimination of restrictions on the use of proceeds from state and local government bond issues.

Elimination of the state’s power to create financial control boards to oversee financially troubled municipalities.

Elimination of limits on real-estate tax revenue used for local purposes.

Elimination of the requirement for state approval of New York City tax hikes.

New levers for state government by restricting home-rule powers and repealing the local government Bill of Rights.

Expanded pension rights and perks for public employees.

Unrestricted abortion rights.

Elimination of the religious-liberty clause.

Guaranteed boosts in financial aid to inner-city schools, particularly failing ones.

Codifying the “prevailing wage,” which requires contractors working on state-funded construction projects to pay workers the amount required by union agreements.

Elimination of charter schools.

Such constitutional revisions would create a state leviathan whose arms would reach into every home and pick the last dollar out of every New Yorker’s pocket.

No, this state can’t afford a constitutional convention that would further empower the tax-and-spend addicts who run Albany and local municipalities.

True, the current Constitution may not be perfect. But New Yorkers know the alternative could have devastating impacts on their financial, economic and social well-being.

Concerned New Yorkers must vote no on Election Day — Nov. 7.

George J. Marlin is a former executive director of the Port Authority.