At the heart of the debate is a question of whether e-cigarettes can be a tool to help smokers quit cigarettes but do so without leading to a glorification of smoking or nicotine use, particularly among young people. The uncertain and sometimes conflicting scientific data comes as traditional tobacco companies – the very cigarette makers that smaller e-cigarette makers hope to displace – have become major players in the new market.

A recent article in The New York Times cited by the senators in their letter outlined how these big tobacco companies have put extensive warnings on their e-cigarette packages. By contrast, many smaller e-cigarette companies have more modest warnings or even advertise health benefits of e-cigarettes that some researchers say are simply unsubstantiated.

On the other hand, some e-cigarette industry officials say the extensive warnings go too far and might be based on ulterior motives by the tobacco companies, perhaps to dampen interest in e-cigarettes — a competitor to their traditional products — or to curry favor with regulators. The companies say that is not the case.

According to the senators’ letter, the F.D.A.’s proposed regulations, which are not final, would include a warning that says: "This product contains nicotine derived from tobacco. Nicotine is an addictive chemical.” The senators say the warnings should be more extensive.

The senators have been among the most aggressive members of Congress in pursuing stronger regulation of e-cigarettes. They were among those signing a House-Senate letter in April criticizing e-cigarette marketing practices that the legislators said were aimed at young people, and they introduced legislation to prohibit such marketing.