About 13% of voters in Wisconsin, Florida and Pennsylvania settled on their candidate choice in the week before the last election, a report by a professional association of opinion researchers found. Those voters broke for Mr. Trump by 30 percentage points in Wisconsin and by 17 points in Florida and Pennsylvania, helping to deliver narrow victories, the study said.

This month, the Journal/NBC News poll went looking for signs of a similar, late shift—and found a different picture than in 2016. Undecided voters seem to be moving equally toward both presidential candidates.

The study wasn’t a formal poll but rather a set of questions put to 184 voters who had answered Journal/NBC News surveys from June through September. In those initial interviews, they had suggested that they were undecided or only tentatively supporting a candidate.

The survey found that many of these voters had picked a candidate in recent months, as the share who were uncommitted fell from 31% to 18%. And they moved almost equally to Mr. Trump and to his challenger, former Vice President Joe Biden. Overall, this set of voters favored Mr. Biden more than the president.