The spinoff of Philip Morris was intended to insulate it from the smoking liability lawsuits and federal regulatory actions that had plagued Altria. With its global reach, Philip Morris is now much larger than its former parent.

Garrett Nelson, a senior equity analyst at CFRA Research, said it might make sense for the companies to reunite, but Philip Morris investors might balk at Altria’s debt load of $29 billion, from its investments in Juul and Cronos, a cannabis company.

“In our opinion, it makes more sense for Altria, because of the ongoing decline of cigarette sales in the U.S. and the heightened regulatory scrutiny for both tobacco and e-cigarettes,” Mr. Nelson said.

Public health officials were less enthusiastic about the prospect of a merger.

“This is very dangerous for public health,” said Matthew L. Myers, president of the Campaign for Tobacco-Free Kids. “There’s a real concern that a strengthened Philip Morris poses an increased threat to tobacco control measures both in the United States and around the globe.”

Scott Gottlieb, a former F.D.A. commissioner, said it was unclear what the merger would mean for public health.

“It’s hard to say it’s a good development,” said Dr. Gottlieb, who initially supported extending the deadline for Juul and other e-cigarette companies to seek agency approval, then regretted it as concerns over youth vaping grew. “One would hope the combined entity would be more focused on truly transitioning smokers off combustible tobacco and onto modified-risk products for adults who still want to access nicotine.”

The F.D.A. has had difficulty grappling with the rise of e-cigarettes. Initially, F.D.A. officials wanted to encourage their development as an alternative to traditional cigarettes, which kill roughly 480,000 Americans a year.