A union-backed ballot measure in California that would limit California dialysis companies from overcharging collected 25 percent of its required signature in four weeks, the campaign announced Monday.

DaVita, the Colorado-based dialysis company headed by Kent Thiry, is among the companies targeted by the ballot measure.

The initiative, known as the Fair Pricing for Dialysis Act, requires companies to refund revenue that is more than 15 percent above the cost of care.

According to the campaign, DaVita and another company, Fresenius, made a combined $3.9 billion in profits across the country in 2016. Private insurance companies paid those companies an average of 345 percent above the actual cost of treatment, the campaign said in a statement Monday.

The California campaign, which began Oct. 30, is led by the Service International Employees Union, United Healthcare Workers West. The ballot measure will need 365,880 signatures to qualify for the 2018 election.

DaVita’s Thiry, a newcomer to Colorado politics, made a big splash in 2016 by putting more than $2.353 million into the campaign for two ballot measures that will now allow unaffiliated voters to cast ballots in primary elections in Colorado. He was rumored to be interested in a run for governor for 2018 but has since backed off, stating he will support “centrist” candidates.