Pets are part of the family, Johnston argued, and families should not have to break up when they hit what he calls a "financial rough patch." He is asking that the federal government modify food-stamp rules to make it easier for low-income people like him to buy food for their pets.
The petition has little chance of succeeding, experts say, given the political and logistical challenges of changing food stamps, otherwise known as the Supplemental Nutrition Assistance Program. But it has attracted the attention of nearly 80,000 signers on the popular petition site Care2, as well as a number of animal welfare organizations.
These groups say allowing food stamps to be used for pet food could potentially keep tens of thousands of animals out of shelters and prevent low-income people from cutting their pets' meals.
"It's potentially game-changing," said Matt Bershadker, the president of the American Society for the Prevention of Cruelty to Animals. "I think we should get behind this in a big way."
Advocates say a food-stamp program that includes pet food would address a little-discussed gap in the social safety net: Currently, there is no federal program that helps low-income people care for their pets.
That population is a large one. According to the National Pet Owners Survey, a poll commissioned by the American Pet Products Association, an industry group, 14 percent of all pet-owning households make less than $25,000 per year - which, for a family of four, is roughly the federal poverty limit.
These households sometimes struggle to cover their pet-related costs, advocates say. Veterinary care and vaccinations are expensive. Food for dogs and cats averages $235 per year, according to the Pet Products Association.
When families don't have enough money to buy pet food, they frequently do what Johnston does: Share the people food. But it's not the same, and it can harm pets. And it can cut into the pet owner's diet - a risk, given that many public health experts say food stamp allotments are already too small to provide adequate nutrition.
Food costs can also prompt low-income families to surrender or re-home a pet. In a 2015 study by the ASPCA, 30 percent of low-income people who relinquished their pets said they would have kept them if they had a free or low-cost pet food option.
About 776,000 of the 5.5 million dogs that enter shelters each year are euthanized, according to Mississippi State University statistics. Towns and cities also spend taxpayer money to round up strays and house them in municipal shelters, though no reliable national estimates exist on the cost of such programs.
"It's hard to know that exact number," the ASPCA's Bershadker said. "But we do know the cost to the animal is potentially death."
The problems are real, but food-stamp experts say it's unclear whether changing SNAP should be part of the solution. SNAP has explicitly excluded pet food since its earliest authorization in 1964, when the Food Stamp Act defined "food" to mean "any food or food product for human consumption." Changing that would require congressional action.
On top of that, there are administrative puzzles involved in extending food stamps to animals, said Craig Gundersen, an agricultural economist at the University of Illinois and an expert on the food-stamp program. (The Agriculture Department declined to comment publicly on the petition.)
"I can't imagine how the government would decide on, say, how much more money a family should get if they have a Bernese mountain dog," Gundersen said, referring to a breed that can weigh more than 100 pounds. "Would this be more than for, say, a Sheltie?"
In lieu of government action, nonprofit organizations such as the ASPCA and Rescue Bank, a national emergency pet-food distributor, say they have stepped up their own efforts. In the years since the recession, concern about pet-food affordability has grown, said John Kane, Rescue Bank's director of development.
His organization distributes donated dog and cat food to animal rescues and shelters across the United States. Recently, more cities have established pet food banks. Food pantries have also gotten in on the action.
PetSmart Charities, the nonprofit arm of pet store chain PetSmart, has teamed up with Feeding America to begin distributing pet food to its national network of food banks. The ASCPA has launched a similar partnership with New York City food pantries.
"As more families struggle with difficult choices like paying the rent or buying food, some have to choose between keeping their pet and putting food on the table," Bill Thomas of Feeding America said in a statement. "We know that pets provide comfort to families and individuals, and those who are struggling financially more than likely will also need food for their pets."
Unfortunately, advocates say, such organizations cannot provide for all the low-income people struggling to feed dogs and cats. Until they can, people like Johnston face difficult choices.
Johnston's town in rural Mississippi is a 2.5-hour drive from the nearest food bank. The closest animal shelter is a county over.
"Being poor is hard enough," he wrote in his petition, "without being expected to give up your companion."