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When Senator Rand Paul of Kentucky kicks off his campaign for president on Tuesday, one of the people who will endorse him and share the stage is former Representative J.C. Watts, one of the few black Republicans to have served in Congress.

Mr. Watts is expected to be a vocal surrogate for Mr. Paul, who has made clear he is trying to show he can broaden the Republican Party’s base to newer voters.

“Senator Paul has shown what I have always believed, a leader can be a strong economic and social conservative and yet be concerned about the least of these,” Mr. Watts, who retired from Congress in January 2003, said in a statement.

Mr. Watts, a college quarterback for the Oklahoma Sooners before serving in the House from that state, has lamented before that Mr. Paul is one of the few members of the Republican Party trying to reach out to voters who have traditionally not been part of the base.

Mr. Watts was a strong supporter of social conservative issues as a congressman. But people close to him say he has admired Mr. Paul’s approach toward growing his base.