Health costs have emerged as the top financial anxiety for U.S. households, according to a new survey. It’s a dynamic that experts say could pose a threat to the Republican majority in Congress during this year’s midterm elections, in which affordability is expected to be a key focus for voters.

Two out of 3 Americans surveyed, 66%, are worried about paying for health care — more than other household necessities such as utilities, food and groceries, housing and rent, and gasoline and transportation, according to a poll published Thursday by KFF, a nonpartisan health policy research group.

Health care includes the cost of health insurance and out-of-pocket expenses for office visits and prescription drugs, for example, according to KFF.

Among those concerned about health costs, 32% said they are “very worried” and 34% are “somewhat worried.” KFF surveyed 1,426 U.S. adults earlier this month.

“This could very well be the No. 1 issue for Americans at the polls in November,” said Nick Fabrizio, a health policy expert and associate teaching professor at Cornell University.