Republicans seeking to overhaul the Affordable Care Act face growing signs that there could be big increases in premiums for individual plans next year, which poses a challenge as the lawmakers try to rally support for the replacement legislation.

According to a nonpartisan report released by the Congressional Budget Office on Monday, the House Republicans’ bill, known as the American Health Care Act, could raise premiums by 15% to 20% for individual plans in 2018, compared with rates without the bill. These increases would largely be due to the end of penalties for people who lack insurance; the CBO suggested that fewer healthy people would enroll without the mandate, helping to raise average costs.

Other factors could also push up premiums and hurt insurer participation next year, insurance-industry officials say. One important element isn’t fully resolved in the House Republicans’ blueprint: whether the federal government will fund cost-sharing subsidies that help pay for low-income consumers’ deductibles and other out-of-pocket charges. Also, the industry officials say, the higher premiums would reflect typical increases in medical spending, including the cost of drugs.

“All this uncertainty, in already a difficult market—the premiums are probably on track to increase significantly again,” said Joan Budden, chief executive of Priority Health, an insurer in Michigan. Priority Health is just beginning to prepare for 2018 rate filings, she said.

A spokesman for the House Energy and Commerce Committee said the 2018 individual-plan marketplaces will reflect Obama administration policies. “Now that we have a like-minded Secretary of Health and Human Services, we know that his actions to provide regulatory relief, taken together with the American Health Care Act, will lower premiums over time,” the spokesman said.