It is the hottest, most expensive campaign in Arizona this year — and it’s not the one for the U.S. Senate.

The battle — which has already cost nearly $54.7 million, or about $11.50 for every eligible voter — is over solar power. A statewide ballot initiative would amend the Arizona constitution to require electric utilities to use renewable energy for 50 percent of their power generation by 2035.

With its abundance of sunshine, that should be an easy reach for Arizona. Yet the state gets only 6 percent of its energy from the sun.

The fiercest foe of the measure? Arizona’s biggest utility. Arizona Public Service, or APS, has poured $30.3 million into a political action committee called Arizonans for Affordable Electricity. In an aggressive ad campaign, the group asserts the measure would cost households an additional $1,000 a year.

On the other side is Clean Energy for a Healthy Arizona, an alliance of about 50 organizations. In its corner is Tom Steyer, the billionaire investor and political activist from California who donated virtually all of the $23.6 million raised through the end of September. Only two Senate races this year, Texas and Florida, will feature more spending.

The battle has been ugly, replete with allegations of fraud, corruption and phony economic analyses.

Beneath the noise, the fight is over two fundamental questions: Are advocates of more solar going to deliver too much of a good thing? Or is a recalcitrant utility standing in the way of economic competitiveness and sensible climate policy?

“Proposition 127 is really a question about which energy future we’re going to have,” said D.J. Quinlan, an Arizona political consultant who is communications director for Clean Energy for a Healthy Arizona.

Arizona has a modest requirement, established in 2006, that utilities rely on renewable energy for 15 percent of supplies by 2025. That target has been met.

“Even at that time the utilities were claiming the sky was going to fall, that they would not able to meet it and the costs were too high,” said Kristin Mayes, a Republican, law professor and former chairman of the Arizona Corporation Commission, the state’s utility regulatory body. “None of that came to pass.”

The effort to boost solar is also tied to the state’s economic competitiveness, Mayes said.

“Many of the very corporations we want to attract to Arizona — like Google, Amazon, Microsoft, Loreal, Apple — have their own 100 percent renewable standards,” she said. “How can Arizona possibly compete for them when we have a utility that says no new renewable energy? That’s impossible.”

The political temperatures have been rising since early summer.

Mayes, who helped write the new measure, said that “unfortunately we are now in a situation where the state’s largest utility has decided to fight solar energy, something overwhelmingly popular in Arizona.”

The APS-backed Arizonans for Affordable Electricity has savaged the “California liberal and activist” Steyer, calling him a “hypocrite” who is “spending millions of dollars to trick Arizonans into voting for a clean energy scheme that will benefit guess who? Tom Steyer.” It also noted that Steyer lives “in a San Francisco mansion.”

Steyer rejects that description. After making money as a hedge fund manager, he has devoted huge amounts of money on promoting climate measures around the country. He has launched an organization, NextGenAmerica, to register young voters. And he has led a signature campaign to impeach President Donald Trump. He is widely expected to run for president himself.

“Their whole approach is to beat up on Tom,” Aleigha Cavalier, a spokeswoman for Steyer, said. “None of it is true. We’re there because Arizona has the biggest potential for solar and to create energy jobs.”

Quinlan rejects the notion that the proposition is the work of people from outside the state. “This is a ballot measure that’s been run by Arizonans and written by Arizonans,” Quinlan said. Among the groups pushing for the plan is Chispa Arizona, an affiliate of a League of Conservation Voters.

Steyer has fired back with his own ads, charging the Arizona political establishment with corruption. He has seized on sizable contributions — at least $4.2 million in 2016 alone — that APS and its parent, Pinnacle West Capital, made to a political action committee that promoted members of the Arizona Corporation Commission that is sympathetic to their view.

Earlier APS and supporters of the ballot initiative clashed over the signatures required to put the measure known as Proposition 127 up for a vote. The coalition collected 437,000 signatures — twice the number needed.

The measure’s supporters said that Arizonans for Affordable Electricity interfered with people gathering signatures, alleging that it tried to pay people to stop doing so.

In July, the utility challenged the signatures in court, alleging fraud and arguing that people had been tricked into signing the petitions. But the Clean Energy for a Healthy Arizona coalition brought 900 people who had gathered signatures to the court to defend their efforts. Maricopa County Superior Court Judge Daniel Kiley rejected the utility’s argument in a 33-page ruling. Even if the proposition is defeated, there is a chance that the utility regulators will demand changes. Andrew Tobin, one of the five commissioners, has proposed requiring a combination of renewable energy and nuclear power for 80 percent of the state’s power sources by 2050, a very long time frame.

Renewables are “good for investors if done properly,” Weinstein says. “And it may be done rapidly because I think costs are coming down faster than anyone expected.”