"It seems to suggest that the health insurance market doesn't have enough opportunities for everyone to participate," Stimpson said. "I think we know from other reports out there that the cost keeps rising so dramatically that many employers may not be able to afford coverage as a benefit to their employees."
The study did not count residents who are 65 and older, who would qualify for Medicaid coverage.
Stimpson said the increase may have slowed in recent years, as parts of the federal health care law allowed children to stay on their parents' coverage up to age 26. But he said the Medicaid expansion is most likely to reverse the trend, as it would extend coverage to an additional 108,000 to 145,000 residents. The law requires states to expand their Medicaid coverage to adults who make up to 133 percent of federal poverty levels, starting in 2014.
The report found that most of the uninsured have incomes above the federal poverty threshold, but one-third do not have full-time, year-round jobs. And an estimated 40,000 of the uninsured are not U.S. citizens.
Stimpson said rates of uninsured were particularly high in the "rural-frontier counties" of western and central Nebraska, regions with larger immigrant populations and fewer companies that can afford to subsidize employee health insurance.
"They tend to be independent business owners or farmers who purchase insurance in a different way, or have fewer options," he said.
Gov. Dave Heineman, who was on a trade mission to China, said Tuesday he hadn't seen the report. The Republican opposes the federal healthcare law but ordered state insurance officials to plan for an online insurance marketplace—where consumers can comparison shop for coverage—to meet the federal requirements.
"If you can reduce the cost of healthcare, you can make it more affordable," Heineman said Tuesday. "That's why we need to focus on prevention, wellness and quality outcomes."
Heineman has previously said expanding Medicaid could force a tax increase or drain money from K-12 education, a network of small state colleges and the University of Nebraska.
The U.S. Supreme Court ruled in June that the federal government cannot penalize states that defy the expansion provision by withholding Medicaid funding that it already gives to states—money that accounts for large a large chunk of the state's health services budget.
Omaha Sen. Jeremy Nordquist, a Democrat who supports the law, said the report highlights the need to extend Medicaid coverage to uninsured Nebraskans to help make coverage more affordable.
"Numbers like this underscore the need to take serious action," Nordquist said. "The problem clearly has been getting worse over the last decade. It's time to address this problem head-on."