SAN FRANCISCO—After Carly Fiorina won California's GOP primary in June, conventional wisdom held she would go after incumbent Sen. Barbara Boxer as many Republicans have before: by tacking toward the center to lure voters beyond the conservative base. Instead, Ms. Fiorina has made a bold and possibly risky bet that pivotal undecided swing voters care more about the economy than social issues and are disenchanted with Washington. So, she is hewing to conservative stands on immigration and social issues, such as abortion and gay marriage, while working to shift the debate to fixing the state's economy. "We are going to...