Chewing tobacco, sometimes called smokeless tobacco, is tobacco that is chewed rather than smoked. Generally, a person chewing tobacco spits the masticated tobacco, now mushed into a liquid, into a receptacle after chewing it. Chewing tobacco does not carry the risk of spreading secondhand smoke as cigarettes do, but passengers still are banned from chewing tobacco on virtually all airlines. However, it is not banned by the Federal Aviation Administration, only by individual airlines.
The Transportation Security Administration, a federal agency that oversees security on flights, maintains a list of all items passengers may not bring on board flights or stow in their luggage. Chewing tobacco has never been on the list, which would mean it is legal to transport chewing tobacco in your carry-on luggage. However, the TSA does not generally regulate activities that are permitted once on the flight; this is left up to the FAA and the individual airlines.
While the FAA does specifically bar individuals from lighting and smoking a cigar, cigarette or any other smoke- or flame-producing object -- as well as all electronic cigarettes -- it does not ban people from chewing tobacco. But virtually all airlines do make it a policy to forbid passengers from chewing tobacco on the flight. However, since airlines are not expressly forbidden from letting passengers chew, you may want to double-check your airline's policy.
If you are bringing chewing tobacco in from another country, be sure to contact the airport in which you are arriving. According to the Department of Homeland Security, each port of entry in the United States has its own rules about bringing loose tobacco into the country. While usually it is fine to bring small amounts in for personal use, large amounts likely will be subject to a tax and other restrictions.
It is harder to detect a person using chewing tobacco on a flight than it is to detect a person smoking a cigarette, as chewing tobacco does not produce telltale smoke and can resemble other permitted activities, such as chewing gum. Some passengers may attempt to chew tobacco in the bathroom to meet their cravings for nicotine. While this still technically is not permitted on flights, it is an activity that is virtually impossible to enforce, as the airlines lack any means of detecting it.