But seniors are also the most vulnerable to the global pandemic, and the campaign’s internal polls, people familiar with the numbers said, show Mr. Trump’s support among voters over the age of 65 softening to a concerning degree, as he pushes to reopen the country’s economy at the expense of stopping a virus that puts them at the greatest risk.

A recent Morning Consult poll found that Mr. Trump’s approval rating on the handling of the coronavirus was lower with seniors than with any other group other than young voters. And Vice President Joseph R. Biden Jr., the presumptive Democratic nominee, in recent polls held a 10-point advantage over Mr. Trump among voters who are 65 and older. A poll commissioned by the campaign showed a similar double-digit gap…

The demographic shift is fairly new, and officials said they attributed it at least in part to Mr. Trump’s coronavirus briefings, at which he often dispensed conflicting, misleading and sometimes dangerous information that caused alarm among a vulnerable population. At the same point in the race four years ago, Hillary Clinton, the Democratic nominee, trailed Mr. Trump by five points with the same group.