And in Washington, White House advisers worked to preserve the law through treacherous politics, sometimes stalling final decisions about the site, HealthCare.gov, to avoid controversy ahead of the 2012 presidential election.

As it becomes clear that no single leader oversaw implementation of the health law’s signature online marketplace—a complex software project that would have been difficult under the best circumstances—the accounts of more than a dozen current and former officials show how a disjointed bureaucracy led to the site’s disastrous Oct. 1 launch…

Politics intervened, too, people familiar with the matter said. Key regulations stalled inside the White House Office of Management and Budget for months while the Supreme Court weighed the constitutionality of the law and the president campaigned for re-election in the 2012 contest.

In one case, a rule governing the design of plans to be sold on the exchange was completed and signed by Ms. Tavenner, the CMS administrator, on May 15, 2012, approved by Ms. Sebelius that Aug. 6 but not released publicly until Nov. 26, three weeks after the election, regulatory records show.

HHS spokeswoman Joanne Peters said many of the law’s major regulations were released before November 2012.