Longtime Republican Rep. Mary Bono Mack has lost her seat to Democrat Raul Ruiz, a Harvard-educated physician who mobilized the district's growing swath of Hispanic voters.
Ruiz led by nearly 7,300 votes Friday out of more than 180,000 cast.
The race in the desert communities near Palm Springs was one of this year's hottest congressional battlegrounds and drew millions in outside spending.
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Bono Mack, an eight-term congresswoman who co-chaired Mitt Romney's California team, acknowledged the race had been her most competitive since she was elected in 1998 to fill the seat of late husband and entertainer, Sonny Bono.
Bono Mack released a statement Friday evening that read, in part:
"Thank you for giving me the distinct honor of serving in the United States House of Representatives for 14 years. Today, I called Dr. Ruiz and congratulated him on his impressive victory. Dr. Ruiz will do a fine job if he is guided as well by the people of the congressional district as I was. Please give him the opportunity to succeed."