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Proposition 8, which started as a fight between a medical workers’ union and private dialysis clinics, has become the subject of one of the most expensive ballot-box campaigns in California history.

More than $110 million has been spent to defeat Prop 8 by a group led by DaVita and Fresenius Medical Care, two for-profit companies that together control 72 percent of the state’s dialysis market in California, the Los Angeles Times reported. Service Employees International Union-United Healthcare Workers has spent $18.8 million in support of the measure, according to the Times.

Related: A Guide to the Propositions on the Nov. 6 Ballot in California, From the Gas Tax Repeal to Daylight Saving Time

The state estimates the nearly 600 dialysis clinics in California make about $3 billion a year offering critical treatment to patients whose kidneys no longer function, with DaVita and Fresenius operating a majority of those clinics.

Voting “yes” means limiting dialysis clinic companies’ revenues, requiring them to refund patients or insurers any revenue that exceeds 115 percent of what it costs to provide drugs and medical supplies, maintain facilities and train and pay staff wages and benefits.

Voting “no” means opposing limits to dialysis clinics’ revenues.

Supporters – SEIU’s United Healthcare Workers Union has spent big for the ballot measure, arguing that it will force companies to hire more staff and upgrade equipments and facilities. The state Democratic Party supports the measure.

Opponents – With dialysis providers paying many millions more than the union to defeat the initiative, campaigns for and against Prop 8 have surpassed the gas tax repeal and rent control as the costliest ballot measure in the California midterm, the Associated Press reports.

The National Kidney Foundation says passing Prop 8 will prompt dialysis providers to close clinics.

Major editorial boards, from the L.A. Times to the Sacramento Bee, contend the labor union behind the measure is using it as a tactic to organize workers at the expense of patient safety. The California Republican Party is also opposed.