CLEVELAND — Donald J. Trump Jr., the eldest son of the Republican presidential nominee, accused the former Trump campaign manager Corey Lewandowski of undermining his father’s White House bid, revealing that reporters told him the former aide was planting stories in an effort to undercut the staff member who has replaced him.

“You can’t claim to be an ally and then try to sabotage the guy that you’re upset that was brought in to work with you,” said Mr. Trump, adding, “I’m not going to allow nonsense to be perpetuated because someone’s looking to get on TV for a few minutes.”

Mr. Trump and his siblings, who are close advisers to their father, were instrumental in Mr. Lewandowski’s ouster. But they had remained publicly circumspect about the aide who, throughout the Republican primary process, was an ever-present figure at the elder Trump’s side.

But long-simmering tensions between Mr. Lewandowski and his successor, Paul Manafort, boiled over this week when Mr. Lewandowski said Tuesday on CNN, where he is now a paid contributor, that Mr. Manafort should “do the right thing and resign” if he was responsible for Melania Trump’s use of language in her Republican National Convention speech lifted from Michelle Obama’s 2008 address to Democrats. Those comments exacerbated a hard day for the Trump campaign, which offered shifting and far-fetched explanations for the plagiarism.