Sen. John McCain chose first-term Alaska Gov. Sarah Palin, little known on the national stage, as his vice-presidential running mate -- a surprise stroke aimed at attracting Hillary Clinton supporters, but which could undercut the campaign's key theme of experience.

Sen. McCain passed over many high-profile, more conventional choices for the job, turning to a woman whose solid conservative positions thrilled the Republican base. Her reputation as a reformer in Alaska dovetails with Sen. McCain's image, too. As a small-town...