The president’s focus on Mr. Obama after about two and a half years in office was even more intense during a trip to Japan and South Korea last weekend, when Mr. Trump repeatedly raised the subject of his predecessor without being asked, assailing him on a variety of domestic and foreign policy fronts.

“When in a corner, Trump falls back on the only organizing principle he has, which is attacking Obama — and usually lying about it,” said Benjamin J. Rhodes, a former deputy national security adviser to Mr. Obama. “I wouldn’t read anything more into it than that.”…

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.