Preserve Our Hawaii, a powerful coalition funded heavily by local unions, has spent more than $600,000 in recent weeks on advertising urging residents to vote “no” in the general election on whether to hold a state constitutional convention, campaign spending reports show.

The question is posed to voters every 10 years. The last convention was held in 1978.

Almost all of the funds have been spent on Core Group One, a Honolulu-based advertising and marketing company, to produce radio, television, print and digital ads.

Preserve Our Hawaii is a diverse group made up of major unions, the Hawaii Democratic Party, the Chamber of Commerce Hawaii, the Sierra Club Hawaii and the Hawaii chapter of the American Civil Liberties Union, among others.

The ballot issue committee, which can spend unlimited amounts, has received contributions from the Hawaii Government Employees Association, the state’s largest union, the Hawaii State Teachers Association, the Hawaii Fire Fighters Association, the University of Hawaii Professional Assembly and National Education Association.

The coalition’s website called “Don’t Be ConConned,” makes three main arguments: that a convention would be too expensive, too dangerous and entail too many unknowns.

Supporters of a convention, by contrast, have argued that a convention would reestablish public trust in government by allowing active participation by citizens, and provide an opportunity for citizens to make reforms that elected officials have shied away from, such as imposing term limits on state legislators and improving campaign finance laws.