Tucson psychiatrist David Ruben has withdrawn from the U.S. Senate race, leaving Richard Carmona as the only Democrat on the Aug. 28 primary ballot.

Ruben turned in enough signatures, 7,842, to initially make the ballot, but he said Friday that after a review of those signatures, he wasn't going to have enough to stay on the ballot. Democratic U.S. Senate candidates need 4,765 valid signatures.

Some of the people who signed his petitions weren't registered to vote, and some didn't write down their complete address, he said. Rather than force his opponent to challenge the signatures in court, he opted to withdraw, he said.

"It's just a shame," said Ruben, adding he should have collected at least 10,000 signatures. "We had a lot of people that worked very hard and are very disappointed."

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Brady McCombs