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Even 36 percent of GOP voters said they would at least consider re-electing him.

Jennifer E. Duffy, an analyst for the Cook Political Report, said Mr. Cardin enjoys relatively strong approval and will have a heavy edge in money and name recognition.

She added that Mr. Muse could have difficulty raising funds and paying for expensive ad campaigns without help from prominent Democrats, most of whom will likely back Mr. Cardin.

“I think he’s in a position to raise some issues and concerns for African Americans,” Ms. Cook said, “but I don’t know if he makes it a really competitive race.”