Allison O'Toole is stepping down as chief executive at the state's MNsure health insurance exchange to work at a nonprofit created by Andy Slavitt, the former UnitedHealth Group executive who held a key health care job in the Obama administration.

O'Toole announced her decision at the start of a Wednesday meeting in St. Paul where the MNsure board named Nate Clark, the current chief operating officer, as acting CEO.

When it was launched as part of the federal Affordable Care Act (ACA) in 2013, MNsure stumbled badly out of the gate with widespread technology problems that outraged consumers and made the exchange a target for Republican attacks. The exchange saw operational improvement and enrollment growth during O'Toole's tenure, but now faces uncertainty as opposition to the health law persists among Republicans at the federal level.

"We're coming off our most successful open enrollment period, helping more Minnesotans than ever before," O'Toole said. "Obviously, that helps our financial position right now. But given the high degree of regulatory uncertainty, we will need to manage our resources with extreme vigilance."

The ACA called for new state-level online marketplaces that are run by the government to provide an option for individuals who don't get employer coverage. Minnesota was in the minority of states to launch its own exchange, which connects many consumers with income-based tax credits from the federal government.

Exchanges across the country face challenges going forward, since the landmark tax bill passed last year by Republicans and signed into law by President Donald Trump eliminates in 2019 tax penalties in the ACA for individuals who lack health insurance. The Trump administration also is pushing rule changes that might let insurers sell policies with limited benefits, a shift that could concentrate individuals with health problems in the insurance market governed by ACA rules.

"We're in the midst of commenting on some of these regulatory proposals that are continuing to undermine the market," O'Toole said Wednesday. She praised bipartisan efforts in Minnesota to stabilize the individual market.

O'Toole is taking a job as director of state affairs with the United States of Care, a new nonprofit with headquarters in Washington, D.C., and Minneapolis that's pushing for better access to quality, affordable health care. Slavitt is the group's board chair. Previously, he was a prominent executive at Minnetonka-based UnitedHealth Group before leading implementation of the health law at the federal Centers for Medicare and Medicaid Services.

"Allison is a proven leader, and we are excited to have her lead our effort to help states cover more of their residents through innovative health care programs," said Emily Barson, executive director of the United States of Care, in a statement.

O'Toole has been the third top executive at MNsure. She became acting CEO in May 2015, before taking the job permanently in November of that year. The exchange saw two chief executives within its first two years of operations.

O'Toole's time as CEO featured challenges, including a difficult start to open enrollment in November 2016 that was marred by alleged "robocallers" jamming the MNsure call center.

In early 2016, the exchange struggled to distribute forms that health-exchange users needed to file their taxes.

But MNsure's operations have improved, as shown by growing enrollment numbers that bucked the national trend.

Enrollment via MNsure of about 58,000 people three years ago has grown to about 116,000 people during the most recent open enrollment period, O'Toole said Wednesday. Over the past four years, she said, average wait times at the MNsure call center have shrunk from 30 minutes to just 10 seconds.

"We all know we did not start off in a perfect place," O'Toole said Wednesday. "But I'm so proud of the work that the MNsure team has done in recent years, and know that they will continue to improve the system in future years."

O'Toole's last day is April 13.

At MNsure, Clark started as the exchange's chief operating officer in December 2016. Previously, he was an executive at Thomson Reuters, most recently a vice president for strategy and business development. As acting chief executive, Clark's annual salary will be $145,000. O'Toole's annual salary was $150,000.

MNsure has an annual budget of more than $40 million and 150 employees.

Figures presented Wednesday by MNsure suggest the number of people paying for health plans sold through the exchange is up this year. The increase helps the MNsure budget, since operations are funded in part by a 3.5 percent tax on policies.

While enrollment via MNsure has been growing for several years now, it remains far shy of original projections. And the state's overall individual market, which includes policies sold directly by carriers, has shrunk significantly since 2014 with premium jumps under the ACA.

Christopher Snowbeck • 612-673-4744 Twitter: @chrissnowbeck