Mr. Trump lashed out in response this month, calling the senator on Twitter a “pompous ‘ass’ who has been fighting me from the beginning.” In one tweet, the president used the hashtag “#IMPEACHMITTROMNEY.”

In the Atlantic profile, Mr. Romney admitted to having what he called a “lurker account” — essentially a profile under a different name that he operated in secret to monitor the political conversation. But he declined to divulge the name associated with the account.

The admission spurred curiosity, particularly that of the online newsmagazine Slate.

Slate theorized that Pierre Delecto, or Twitter user @qaws9876, was Mr. Romney after it discovered the account among the Twitter followers of one of his grandchildren.

Slate’s Ashley Feinberg found that Pierre Delecto’s first follower was Mr. Romney’s oldest son, Tagg. The account was created in 2011, shortly after Mr. Romney announced his intention to run for president. The account also followed all of Mr. Romney’s children who are on Twitter and several former advisers, according to Slate.

The account also posted several telling replies to Romney-related tweets, which were captured by screenshot before the account was made private on Sunday night.