Prop. 127 would create a constitutional provision requiring that half of all electricity come from renewable sources.

PHOENIX — At about $40 million so far for both sides, Proposition 127, the renewable-energy mandate, is shaping up to be the most expensive ballot fight in Arizona history.

California billionaire Tom Steyer has spent more than $18 million so far trying to persuade Arizonans to support a constitutional mandate that half of all electricity come from renewable sources by 2030. Steyer, through his NextGen Climate Action Committee, has put that money into getting the measure on the ballot and promoting it.

On the other side of the equation, the parent company of Arizona Public Service, the state’s largest electric utility, has poured $21.8 million into quashing the measure.

APS isn’t alone. Rural electric cooperatives have put $417,000 into their own effort against the initiative. And UniSource Energy Corp., parent of Tucson Electric Power, has a separate $50,000 campaign against Proposition 127.

All that eclipses the record set in 2002 when Native American tribes put $21.1 million into a successful initiative campaign to give them the exclusive right to operate casino-style gaming in Arizona in exchange for the state getting a share.

That same election the Colorado River Indian Tribes spent $10.3 million in their failed effort to get voters to adopt a different measure that would have provided more types of gambling and less revenue sharing with the state. Horse and dog track owners spent $7.7 million that year supporting their own proposal, which would have allowed them to have slot machines as well.

Renewable energy isn’t the only thing commanding a lot of campaign cash this year.

The Arizona Association of Realtors and its national organization reported it had built up a war chest of $8 million in its support of Proposition 126.

That measure seeks to alter the state constitution to make new sales taxes on services permanently off limits. Proponents, operating under the banner of Citizens for Fair Tax Policy, are running TV ads.

A newly formed No on Proposition 126 committee had not collected or spent anything as of the end of the campaign filing period.

The other hot-button issue on the ballot has to date not gathered anywhere near the dollars of the first two.

Save Our Schools has so far spent $426,000 on its bid to get Arizonans to vote “no” on Proposition 305.

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This measure is a referendum of 2017 legislation which would expand who is eligible to get vouchers of state dollars to send their children to private and parochial schools.

A group called Yes for Ed AZ has spent about $26,000 so far to convince people to vote “yes” and allow the voucher expansion to take effect. It is being financed largely by the Center for Arizona Policy and the Goldwater Institute.

On the other side, opposing more vouchers, is the Save Our Schools campaign committee, has spent $426,000. There was no immediate response to a request for a list of major donors to that organization.

Proposition 306, by comparison with the others, has gathered relatively little publicity and relatively few dollars.

One of its provisions would bar candidates who get public funding from using those dollars to buy services from political parties. Another would subject the rules adopted by the bipartisan Citizens Clean Elections Commission to oversight by the Governor’s Regulatory Review Council.

Proponents have collected $10,500, most of that from the Arizona Free Enterprise Club, which has refused to disclose its donors.

Foes have organized under the name of Arizona Advocacy Network Against the Dark Money Lobbyist Power Grab. They have collected slightly more than $4,900, with more than half coming from Tom Collins, the executive director of the Clean Elections Commission.