A bankruptcy judge has refused to shield what is left of Donald Trump’s former Atlantic City, N.J., casino empire from union protests.

Judge Kevin Gross said in a decision issued Tuesday that when it comes to a campaign meant to warn potential customers of the Trump Taj Mahal that workers are unhappy, federal labor laws protect the workers. The ruling comes after months of street demonstrations led by Unite Here Local 54, the union that represents workers that lost health care and other benefits due to the financial distress...