Both sons have operated and promoted the Trump family business overseas during their father’s presidency, even as he retains ownership. And while the Trump and Biden father-son relationships differ in many ways, the business dealings have set up a simple parallel.

“They are criticizing the vice president’s son for doing exactly what they are doing themselves,” said Martin Ford, a member of the Aberdeenshire Council in Scotland, which voted last month to approve a proposal by the Trumps to build a 500-unit housing development. “They are conducting international business here in Scotland.”

Eric Trump, in a statement to The New York Times, said there was a distinction between his father’s career in business, with his recent turn to politics, and Mr. Biden’s decades in public office.

“When my father became president, our family stopped doing international business deals,” he said, a reference to a pledge by the Trump Organization not to begin new overseas projects during the Trump presidency. Hunter Biden, he said, did the opposite when his father became vice president.

The Trump business activities in India, Indonesia and Scotland were considered in compliance with the pledge, according to the company’s ethics rules, because they were building on deals already in the works.

Mr. Biden’s campaign declined to comment on Friday. But in a tweet, the former vice president once again suggested that the attacks on his son were part of a political smear campaign. “Let me make something clear to President Trump: I’m not going anywhere,” he wrote. “You’re not going to destroy me. And you’re not going to destroy my family. I don’t care how dirty the attacks get.”