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Neither candidate managed to get the 51 percent needed to secure the seat. Ridley-Thomas got 45 percent of the vote, while Parks got 40 percent.
With no clear winner in Tuesday's primary, both candidates must now slug it out in a November runoff.
Ridley-Thomas jumped to an early lead as election returns were announced Tuesday night, and he never relented.
"You know how to work, you've proven it tonight. And that's why we stand tonight with a sense of pride, a sense of purpose and a lead that is respectable and can make it all the way because of you and what you've done. Give yourselves another big round of applause," said Ridley-Thomas.
Even though Parks has raised more money than Thomas in the fundraising arena, Parks maintains he's done it one buck at a time.
"All of us in elected life would like to have a campaign one day where somebody says here's $5 million go run a campaign. We didn't get to do that. We collected the money the old-fashion way, one dollar at a time. From each and every one of you, we collected it to where there is no special interest. The only interest is from the community itself," said Parks.
Both candidates will hit the campaign trail again for another five months.
The candidate elected to the 2nd District seat, currently held by retiring Supervisor Yvonne Burke, will become the first African American ever elected to the County Board of Supervisors.