At the same time, public support for the perennially controversial law has inched up to around its highest point in a half-dozen years. Nearly 9 million people so far have signed up for ACA health plans for 2018 during a foreshortened enrollment season, far surpassing expectations.

This dual reality puts the sprawling ACA — prized domestic legacy of the Obama era, whipping post of the Trump administration — at a new precipice, with its long-term fate hinging on the November midterm elections certain to consume Washington once the new year begins. If Democrats win a majority in either chamber of Congress, the law would be protected; a GOP sweep could further embolden repeal attempts.

“It’s right on the balance there,” said Robert J. Blendon, a Harvard professor of health policy and political analysis. “The viability of the program is heavily dependent on the outcome of the election, not the changes in between.”