An anti-Trump group produced four ads attacking his legal troubles. They backfired.

In late September, a conservative group opposing Donald Trump quietly tested four TV ads that aimed to weaken the former president by focusing on a central issue of the campaign: His myriad legal troubles.

One spot, which was surveyed before an online panel of Republican primary voters, declared that the indictments against Trump had “worn” him “down” and undercut his ability to win the election. Another said the trials presented “too much baggage” and warned that Democrats would “sensationalize” them to hurt the ex-president. The hardest-hitting commercial raised the specter...