Democratic presidential frontrunner Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE knocked airlines this week for charging high ticket prices despite decreases in jet fuel and the price of oil in recent months.

"Over the past year, oil prices have fallen from over $100 a barrel to under $50, and the price of jet fuel has dropped more than a dollar per gallon," Clinton wrote in an op-ed published on Quartz Magazine's website titled "Being pro-business doesn’t mean hanging consumers out to dry."

"But the four major airlines—down from 10 airlines just 15 years ago—are charging as much as ever for tickets, even as they hit travelers with extra fees, for everything from checking a suitcase to picking a seat when they fly home at the holidays," the former secretary of State continued.

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Consumer protection groups have complained for years about airline ticket prices. The Department of Justice said earlier this year that it is investigating whether airlines are colluding to keep fares artificially high after a string of mergers has greatly consolidated the industry.

Airlines have denied the collusion allegations.

The group lobbying for airlines, Airlines for America, said Clinton is wrong about airfare prices being on the rise.

"Fact: airfare has fallen 3.9% since last year,@HillaryClinton. Comparably, air service is one of the best bargains," the group tweeted.

Fact: airfare has fallen 3.9% since last year,@HillaryClinton. Comparably, air service is one of the best bargains. https://t.co/M8dR9XLILf — Airlines for America (@AirlinesDotOrg) October 21, 2015

"The facts do not support the assertion," the airline group said in a statement that was provided to The Hill. "Airfare remains one of the best consumer bargains out there, and during the first eight months of 2015, airfare has actually fallen 3.9 percent from the same period last year, including 2.8 percent domestically. Also the average domestic airfare fell nearly 7 percent in August year over year."

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Clinton cast her comments about airlines in her op-ed as part of her commitment as a presidential candidate to fight for middle class U.S. residents.

"American capitalism built the greatest middle class in history," she wrote."When it works the way it should, our system is defined by innovators constantly sparking new ideas, workers sharing in the profits they help produce, consumers enjoying ever-greater choices, and small business owners like my father, working hard to give their families a better life. But sometimes, the system doesn’t work the way it should and we need to fix it.

"Teddy Roosevelt had to do it. Franklin Roosevelt had to do it. Barack Obama, too," Clinton continued. "Despite all the progress we’ve made coming back from the financial crisis, we still have a lot of work to do."

Consumer groups cheered Clinton for speaking out on behalf of airline passengers.

"The article … by Hillary Clinton directly and indirectly shines a bright light on the problems of U.S. airline industry concentration and resulting anti consumer and anti competitive policies and practices," Business Travel Coalition founder Kevin Mitchell and Travelers United President Charlie Leocha said in an email.

"We believe these behaviors conspire against the interests of consumers and rightly belong in the U.S. national presidential debate," the passenger advocacy groups continued.

The airline group said airfares are set fairly by U.S. carriers, however.

"Our members compete vigorously every day, and the traveling public has been the beneficiary, as average domestic fares have declined in 2015," the group said.

"It is customers who decide pricing, voting every day with their wallets on what they value and are willing to pay for," the group continued. "Because there so many options among air carriers, with low cost carriers being among the fastest-growing market segment, more people can afford air travel, as evident by the fact that this summer saw the highest number of people flying since the recession."

This story was updated with new information at 10:22 a.m.