“So the end result is, we’ll see,” Mr. Trump said last week, again broaching the summit meeting with North Korea.

At least two dozen times in the past month, the president appears to have shifted into full-on verbal tic mode, deploying some variation of “We’ll see what happens” as a cast of world leaders from France, the Baltic States, Japan and Nigeria rotated in and out of the White House or his Florida estate. Mr. Trump’s one-person guessing game came into play as he addressed topics including Mexico, Nafta, Russia, Amazon, North Korea, Mike Pompeo, Dr. Ronny L. Jackson, Iran and the special counsel’s investigation into his presidential campaign.

Republican and Democratic speechwriters and others who study the president’s speech patterns say “we’ll see what happens” may be a way to signal a veiled threat to unpredictable adversaries, like the North Korean leader Kim Jong-un, amid delicate negotiations. But many who watch closely say Mr. Trump is using the phrase to avoid accountability.

“The occasions in which he’s made specific promises, like ‘we’ll build a wall and Mexico would pay for it,’ he has had trouble delivering,” Kathleen Hall Jamieson, the director of the Annenberg Public Policy Center at the University of Pennsylvania. “Instead of forecasting and being accountable for the forecast, he’s opening the possibility that there are a range of possibilities not anticipated for which he does not want to be held accountable.”

Mr. Trump is not the first president to rely on a verbal crutch to make a point, avoid criticism or skirt a question. When faced with pressure, President George W. Bush famously referred to himself as “the decider” — a phrase thought of as the “You’re not the boss of me!” of presidential declarations — and had a penchant for using “fabulous,” a decidedly non-Texan adjective, to voice his approval. President Ronald Reagan tended to tug on his ear and mutter “I can’t hear it” to avoid questions shouted at him on the way to Marine One. And President Barack Obama often relied on the phrase “let me be clear” to get his audiences to pay attention during several of his first-term speeches, until people started to catch on to the rhetorical device.