President Donald Trump’s daughter and son-in-law, both top White House aides, are threatening to sue a group of anti-Trump Republicans for posting billboard ads in New York City’s Times Square linking them to the country’s almost 225,000 coronavirus deaths.

A lawyer representing Ivanka Trump and her husband, Jared Kushner, made the threat on Friday in a letter to the Lincoln Project, calling the ads “false, malicious and defamatory” and saying they constituted “outrageous and shameful libel”...