Rejected twice by voters in the past two years, a Lake County tax measure aimed at combating algae blooms in Clear Lake and protecting it from invasive mussels will return on the November ballot.

The proposed half-cent sales tax measure, backed by business, agriculture, political and environmental leaders, would raise $24 million over 10 years, with every dollar committed to protecting the 68-square-mile lake that is the county’s economic lifeblood.

“The lake affects pretty much everything in Lake County,” said Scott Knickmeyer, chairman of the Save the Lake Committee. “We feel we need to give it another shot.”

The Board of Supervisors voted unanimously Tuesday to put the measure on the Nov. 4 general election ballot, hoping the “third time is a charm,” Supervisor Anthony Farrington said.

As Measure L on the June 3 primary ballot, the tax fell 220 votes short of the two-thirds majority needed for a tax with a specified purpose, garnering 65.2 percent voter approval. In November 2012, a similar measure fell 800 votes short, with 63 percent support.

“We know it’s a tough battle,” Knickmeyer said of the mandated 66.7 percent approval. “We just feel like now is the time,” he said, to build on the campaign mounted earlier this year rather than waiting until 2016.

Backers put up nearly $136,000 for Measure L, with more than 80 percent coming from the Lake County Association of Realtors, headed by Knickmeyer.

In a letter to the supervisors two weeks ago, Save the Lake Committee member Anita McKee said there had been considerable comment from “many, many people who were confident that the measure would pass and are now deeply concerned that it didn’t. We believe we can count on these people to help us get the word out.”

The proposed tax would provide Lake County’s $8 million contribution to a $48 million federal project that would breach levees at the north end of the lake and restore the 1,400-acre Middle Creek area to its natural state, covered by water up to 8 feet deep.

That expansion of the lake would absorb about 40 percent of the phosphorus that flows in from two creeks, feeding the algae that periodically create unsightly, stinking masses in the warm, shallow lake.

Tax funds also would support measures to protect the lake against an invasion of quagga mussels that have wreaked havoc in waterways across the nation and made their way into nearly two dozen Southern California lakes and reservoirs.

Knickmeyer said the pro-tax campaign will emphasize that the county cannot afford the restoration project without new tax revenues and that an advisory committee appointed by the supervisors will meet publicly and monitor use of the funds.