OAKLAND -- Alameda County voters Tuesday renewed a half-percent sales tax that has been bolstering the health care safety net for the past decade.

Measure AA required 66.7 percent support to pass and was winning by nearly 75 percent, though 64,000 absentee ballots were still to be counted. The tax renewal measure sought to extend for another 15 years an annual revenue stream of more than $100 million that funds hospitals and community clinics serving indigent, uninsured and low-income Alameda County residents.

It was designed to succeed Measure A, the temporary half-cent sales tax that 71 percent of county voters approved in 2004. Measure A funding expires in 2019, but Measure AA asked voters to extend the tax through 2034.

If the measure failed to win, backers were already considering trying again in 2016. They had little time to rally a countywide campaign but raised more than $650,000 for the cause after the Alameda County Board of Supervisors voted in February to place the measure on the June ballot. It appeared to work, winning a greater margin of victory than the original 2004 measure.

By Wednesday, poll workers had counted nearly 95,000 votes in favor of Measure AA and 32,000 against it. Still to be counted, however, were 64,000 mail-in ballots, most of them dropped off at precincts Tuesday, as well as 8,000 provisional ballots. Final results are not expected before next week, though they are unlikely to change the outcome, said Tim Dupuis, the registrar of voters.

Some backers initially opposed taking the measure to voters so soon but changed their minds. Among the reasons to hurry was a competing countywide transportation sales tax measure on the November ballot, promising results from a consultant's poll and the hope that if the measure failed the first time, supporters gave themselves plenty of time to try again before 2019.

Back in 2004, the measure won overwhelming support of more than 80 percent in Berkeley, Albany and Oakland -- home to the county's flagship public hospital -- but a narrower 53 percent majority in the Tri-Valley, where the benefits of the county public health system were less obvious but where campaigners focused much of their attention this year.

Turnout was low Tuesday, just 25 percent of all registered voters in Alameda County, Dupuis said.

As with Measure A, three-quarters of Measure AA revenue will go directly to the operating budget of the Alameda Health System, the public consortium that runs Highland Hospital in Oakland, along with John George Psychiatric and Fairmont hospitals in San Leandro and three community clinics in Hayward, Newark and Oakland. The consortium, formerly known as the Alameda County Medical Center, also took over San Leandro Hospital last year and Alameda Hospital in May.

The balance will continue to be allocated by the Alameda County Board of Supervisors, which has used the money to fund an array of community-based primary care clinics, school health centers, jail mental health services and other safety net programs. Staffed by bilingual workers, many of the hospitals and clinics treat immigrants denied health insurance because they live in the country illegally, though they also serve many others.

The measure won endorsements from health care unions, physicians, business groups and city councils. Backers emphasized in mailers and a television commercial that the proposal "does NOT" increase taxes, only renews the existing sales tax of a half-cent for every dollar spent. Big campaign donors included the Service Employees International Union, Kaiser Permanente and groups and individuals representing local health providers that directly benefit from the tax revenue.

There was no organized opposition and only one person, Oakland resident David Mix, wrote an argument against the measure that appeared on the voter pamphlet. Among the concerns were lack of oversight over how the sales tax money is spent and a discretionary fund that allows each county supervisor to spend $150,000 a year on chosen health programs.

Critics also said that before extending the tax, the county should get a better understanding of how local hospitals and clinics will be affected by the federal Affordable Care Act's recent expansion of Medi-Cal coverage for low-income Californians and an associated drop in state funding.