Democrat Hillary Clinton’s presidential campaign on Monday canceled an event in New Jersey to spend more time campaigning in California this week ahead of the June 7 primary, hoping to avoid an embarrassing defeat to rival Bernie Sanders, who has barnstormed the Golden State.

Mrs. Clinton had been scheduled to campaign in New Jersey on Wednesday and Thursday. Her campaign said Monday that instead, on Thursday she would begin a five-day California swing. Some recent polls have found the race nearly tied, though others put...