“These products threaten to undo all that,” said Matthew L. Myers, president of Campaign for Tobacco Free Kids, who wants the F.D.A. to issue tough guidelines for how e-cigarettes can be marketed and sold. With such regulations, “e-cigarettes have potentially positive value,” he said, “but we’ve already seen that, if left to their own, e-cigarette manufacturers will reach out to our children, do everything to maximize sales, including re-glamorizing smoking, and that’s where we are today.”