A Pennsylvania judge declined Wednesday to block the state's voter-identification law from taking effect, saying opponents would likely fail to show the law violates the state's constitution.

Opponents of the Republican-backed measure, which requires Pennsylvania's 8.2 million registered voters to present a state-approved photo ID, such as a driver's license, at the polls, said they planned to file an appeal Thursday to the state Supreme Court. Legal experts said it is unclear if an appeal would be ruled on before the November...