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The race to succeed Senator Harry Reid in Nevada has its first candidate.

Catherine Cortez Masto, a Democrat and former attorney general in Nevada, announced Wednesday that she was running for the seat of Mr. Reid, who is retiring next year.

“I’m running for the Senate to continue my work standing up for Nevada seniors, consumers, homeowners, women and children,” Ms. Masto said, asking for donations to help combat an expected flood of outside money and attacks.

Ms. Masto, 51, is Mr. Reid’s preferred successor and would be the first Latina elected to the Senate. She was elected attorney general in 2006 and served two terms, but was barred from running again by term limits.

The Republican field remains less clear, with Michael Roberson, a State Senate majority leader, expected to be in the mix.

The Democratic Senatorial Campaign Committee endorsed Ms. Mastro on Wednesday, calling her “the strongest candidate to keep this Senate seat in Democratic hands and continue Harry Reid’s legacy of fighting for Nevada’s best interests.”