Thirty-four members of Congress are calling on the Federal Trade Commission to investigate reports of widespread overcharging by some of America’s most-recognized brand-name retailers.

The 28 October letter to the FTC comes in direct response to Guardian investigations of four of the nation’s largest dollar-store and convenience-store chains. Both investigations revealed pervasive mismatches between shelf labels and register prices.

The first, published in December, found that Dollar General had failed more than 4,300 government price-accuracy inspections in 23 states over a three-year period on items such as peanut butter, spaghetti sauce and Flintstone vitamins. Its smaller rival, Family Dollar, failed 2,100 price inspections in 20 states over the same period.

The second, published last month, found a similar pattern at convenience store chains 7-Eleven and Circle K. Consumer complaints, along with state and local inspections, revealed overcharges for snacks, beverages, toiletries, tobacco and medications. In addition, some stores charged more at the pump for gasoline than the prices displayed on their big road-facing signs.

“These reports show that these incidents are not isolated to a geographic area or to a handful of stores,” the Congress members’ letter said. “Instead, the scale of documented violations seems to be the result of systemic failure within corporate operations, leading to customers overpaying for their groceries, potentially without knowing.”

All four companies have told the Guardian in written statements that they try to keep register prices consistent with shelf prices.

The letter, written by US House Democrats from 17 states and the District of Columbia, was addressed to FTC chair Andrew N Ferguson, a Trump appointee.

It was sent two days after the FTC, along with 20 states, filed a federal lawsuit accusing Dollar General of a “pattern of inaccurate pricing” that “affects millions of purchases”. The lawsuit claims the company knew about the problem for years without correcting it.

Dollar General, in a statement, vowed to “defend this litigation vigorously”, saying it aims for 100% price accuracy and that it cooperated with the FTC’s investigation.

“While this step is positive,” the lawmakers wrote of the FTC’s lawsuit, “the widespread nature of the issue revealed by public reports and state inquiries warrants a full-scope probe.”

The congressional letter comes on the eve of a midterm election in which affordability has emerged as the major concern. In a recent Harris Poll conducted for the Guardian, voters listed gas prices, grocery prices and living expenses as their top three election issues.

In another poll, released by AP-Norc Center for Public Affairs Research, nearly 80% of voters disapproved of Donald Trump’s handling of the cost of living.

“Gas prices are at $4.50 a gallon in my district,” US representative Nikki Budzinski of Illinois, the letter’s lead author, told the Guardian. “Any uptick in a gas price, or any uptick in the price of a banana or the price of ground beef, means families, in reality, making very difficult decisions as to whether they’re even able to afford that item any more … They’re making decisions as to whether to fill their gas tank up completely or fill their grocery basket up.”

Back in December, Budzinski and 29 colleagues wrote to the CEOs of Dollar General and Family Dollar seeking internal documents about their pricing disparities. Neither company complied, she said, “so we’re taking the next step of going to the FTC and asking them to do enforcement.” The federal agency is charged with policing unfair, deceptive and anti-competitive business practices.

The Guardian has contacted the FTC and the four retail chains for comment.

Budzinski and her co-authors noted that many of their constituents live too far to walk or drive to independent grocery stores or chain supermarkets. A Dollar General or 7-Eleven might be the closest place for them to buy food.

“Because these stores are often the sole option for essentials in underserved neighborhoods and rural communities, these companies occupy a unique position of trust and responsibility,” the letter said. “It is deeply troubling to see that these limited options have been exploited to the detriment of working families.”

For one of the lawmakers who signed the letter, the issue is personal. US representative Shontel Brown lives near a Dollar General in Warrensville Heights, Ohio. She visited earlier this year to buy some soda pop. When she reached the register, she said, the total cost didn’t match the advertised price.

“I asked the cashier, ‘Wait a minute, how am I paying X amount when these items are on sale and I’ve applied these coupons?’” Brown recalled. “And she said, ‘Well, the system does that. It does it a lot with the pop.’”

The congresswoman left without making the purchase. “I have the luxury of not counting every single dollar in my budget,” she said. Many of her constituents don’t have that leeway. “I can’t imagine folks who are coming in there, who have to itemize every dollar, dealing with something like this. So when the letter was brought to my attention, I was a ‘hell yes’.”