Gas prices could decide the midterms. Here's why.

Midterm election results may come down to the price of a gallon of gas.

Roughly half of Americans say either the economy or inflation is the most important issue in their vote for Congress, making bread-and-butter financial issues by far the most dominant in the lead-up to the midterm elections, according to an ABC News/Ipsos poll released on Sunday.

Meanwhile, research has established a relationship between gas prices and presidential approval ratings that reaches back decades, despite the limited control that presidents exert over fuel costs, experts told ABC News. Presidential approval ratings, in turn, mark a key indicator of midterm success or failure for the party in control of the White House, they said.

After reaching a summer peak, gas prices declined for about 100 consecutive days, buoying Biden's approval rating. But a price spike in early October weighed on his approval.

Over the last few weeks, prices have returned to a steady decline – though they remain elevated – leaving an open question about whether consumers will recognize the trend and reward the Democrats or punish them for too little, too late.

"Swing voters will be deciding as they're literally walking into the voting booth," Colin McAuliffe, co-founder of left-leaning research firm Data for Progress, told ABC News.

The ubiquity of gas prices emblazoned on towering roadside signs can affect the financial attitude of those who do not even purchase gas. For those who do, filling up the tank offers a few minutes of repose with little to do but watch the price slowly add up, the experts said.

"People know their grocery bill has gone up but they can't say necessarily how much the price of meat versus milk versus cereal has changed," Laurel Harbridge-Yong, a professor of political science at Northwestern University who has studied the political implications of gas prices, told ABC News. "Gas is a very visible item and an item you buy one at a time, without bundling it with others."

In 2016, a study in the academic journal Political Psychology examined the relationship between gas prices and presidential approval rating between the mid-1970s and mid-2000s, finding that elevated gas prices drove a president's approval downward. To be exact, each 10-cent increase in the gas price was associated with more than half a percentage point decline in presidential approval, the research showed.

Further, the researchers studied press coverage of the gas prices, finding that the effect on presidential approval occurred regardless of how much attention the prices got. On that score, gas prices contrast with other economic indicators, like the unemployment rate or overall inflation, which typically require media coverage that gives voters a sense of the trend, said Harbridge-Yong, one of the researchers who conducted the study.

McAuliffe, of Data for Progress, has established more recent findings, demonstrating a correlation between gas prices and presidential approval during Biden's time in office stretching to as recently as August. "The correlation has held up pretty strongly," he said.

The continued salience of gas prices amid inflation under Biden comes as no surprise to ​​Jon Krosnick, a professor of political science at Stanford University who co-authored the 2016 study. "When we see gas prices go up as much as they have gone up, clearly there are implications," he said.

Despite the relationship between gas prices and presidential approval rating, presidents exert little control over fuel costs, leaving them largely powerless to address perceptions of their performance in this area, experts said.

The U.S. is set to produce an average of 11.8 million barrels oil per day in 2022, which stands 500,000 barrels short of a record set in 2019, according to the U.S. Energy Information Administration. But oil prices are set on a global market, where supply shortages caused by the Russia-Ukraine War and OPEC+ output cuts cannot be offset by a comparable short-term increase in U.S. oil output.

Typically, gas prices drop ahead of midterm elections, since the Fall brings a decline in demand as Americans scale back from summer travel, said Patrick De Haan, an oil and gas analyst for GasBuddy. In recent weeks, the drop has also stemmed from the repair of damage at a string of oil refineries, which brought them back online and increased overall output, he added.

"It's normal for prices to go down this time of year," De haan told ABC News. "It's not political."

The price relief may have arrived too late for voters to notice, said Krosnick, of Stanford University.

"There's not a lot of time between now and election day," he said. "Obviously any change in gasoline prices has to take place and be detected by the public."

Biden highlighted the issue on Monday, threatening oil and gas companies with higher taxes if they do not relieve a supply shortage with increased output. Many of the major oil producers have reported recorded profits in recent quarters.

On election day, gas prices will play an unmistakable role in the outcome, Krosnick said.

"People bring to the table a mix of issues. You can think of it as making a complicated soup with lots of ingredients that play a part," he said. "Gas prices clearly are a part of the soup."