In a debate a month ago, he declared himself in favor of torture if it would extract information from terrorists, then issued a statement saying he would respect the law, then followed it up by saying that the law must be changed.

In a television interview on March 30, he suggested that women who have illegal abortions should face “some form of punishment,” then reversed himself in another carefully worded statement, then suggested in an interview that abortion should remain legal, then effectively clicked “undo” with another written statement.

In each case, Mr. Trump appeared to be seeking a sort of shelter somewhere between his original remark and the wave of objections it set off.

None of this seemed to matter much until recently. But the Trump style — long on gut instincts, short on briefing books — has taken a toll. His opponents have called him reckless and unfit to be commander in chief. Mr. Obama has said Mr. Trump “doesn’t know much about foreign policy, or nuclear policy, or the Korean Peninsula, or the world generally.”