Since then, Mr. Trump has been determined to minimize or unravel Mr. Obama’s accomplishments, and lately has even suggested that his predecessor was behind a deep-state conspiracy with law enforcement and intelligence agencies to thwart his 2016 candidacy.

While other presidents have blamed their predecessors for various national ills — including Mr. Obama, who in his first term regularly pointed to former President George W. Bush — Mr. Trump takes it further than most.

It is less common for presidents to take on predecessors who are more popular than they are; Mr. Obama was viewed favorably by 63 percent of those surveyed by Gallup last year, while Mr. Trump’s job approval rating is 41 percent.

But Mr. Trump recognizes that his political base wanted, and still wants, someone who would be seen as fighting against Mr. Obama. Especially as Mr. Biden stumps the country on his record in the Obama administration, Mr. Trump sees a political advantage in taking down his predecessor and trying to lift himself as an outsider taking on a system he has led for over two years.

“Tell Biden that NATO has taken total advantage of him and President Obama,” Mr. Trump said on Friday. “Biden didn’t know what the hell he was doing and neither did President Obama. NATO was taking advantage of — now they’re paying.”

“President Obama and Vice President Biden,” he added, “they didn’t have a clue. They got taken advantage of by China, by NATO, by every country they did business with.”

By Mr. Trump’s indictment, Mr. Obama was too soft on China’s trade abuses and too easy on NATO allies who were not spending enough on their own defense, two issues that the current president has pressed much more vigorously. Mr. Trump in recent days has also blamed Mr. Obama for a dispute with Turkey, a NATO ally, over its purchase of S-400 missile systems from Russia. A former Obama aide denied that he refused to sell a Patriot system to Turkey but did object to a technology transfer Ankara demanded as part of a deal.