But in a marked contrast, two-thirds of the marketplaces run by states instead of the federal government saw increased enrollment between 2017 and 2018, including Connecticut and New York.
That in large part reflects extensive efforts in many of these states to market health coverage, lengthen enrollment periods and conduct aggressive outreach campaigns to attract younger, healthier consumers who are critical to insurance markets.
"We had the best open enrollment period we have ever had," said Allison O'Toole, chief executive of Minnesota's insurance marketplace, known as MNsure, which saw enrollment surge nearly 6 percent this year. Elected officials in Minnesota developed their own reinsurance system to help control premiums this year.
Even bigger enrollment increases were recorded in Washington state, which had 7.6 percent increase, and Rhode Island, which led all states with a 12.1 percent increase.
HealthSource RI director Zachary Sherman attributed the success to the state's ability to manage its own market, craft marketing and work with insurers to control premiums.
California's enrollment dipped slightly in 2018, falling 2.3 percent, but the state saw a significant increase in new consumers, which was an encouraging sign, said Peter Lee, head of Covered California, the state's marketplace.
"Put simply, marketing matters," Lee said.
That is reflected not only in enrollment totals, but also in the type of consumers that state marketplaces were able to attract.
California, for example, has consistently had a healthier mix of enrollees than markets in other states. That, in turn, has helped keep premiums in check compared to other states and has kept markets competitive, with multiple insurers selling plans in most parts of California.
While California and other states that operate their own marketplaces invested in marketing and outreach, the Trump administration took a number of steps that weakened enrollment.