The FBI committed “serious performance failures” in 2016 when it sought to monitor a former Trump campaign adviser, an internal watchdog said, as part of what it found to be a justified probe into links between the Trump campaign and Russia.

The report’s scathing assessment of the FBI’s actions makes it likely to further inflame a partisan debate about the propriety of the agency’s investigation into Trump campaign associates. The Justice Department’s inspector general found the FBI had information that undercut material...