Key U.S. official faces grilling over how Obamacare website became fiasco WASHINGTON (Reuters) - Two months before the troubled October 1 launch of Obamacare exchanges, a key administration official overseeing the program assured a congressional oversight panel that work was on track to roll out a tested website that would make it easy for Americans to enroll in affordable health insurance coverage. "This is a large and complicated endeavor that I am proud to lead, and every decision is being made by my prior work experience," Marilyn Tavenner testified on August 1 before the House of Representatives Energy and Commerce Committee, describing the launch of the Healthcare.gov website. Come Tuesday, the former nurse who heads the U.S. Centers for Medicare and Medicaid Services (CMS) will again find herself before a House committee - this time, to explain how Healthcare.gov failed when the administration flipped the on switch. She will face Republicans eager to prove, thus far unsuccessfully, that the White House orchestrated decisions that may have stalled the system. Lawmakers on both sides of the partisan aisle are growing increasingly impatient with website snafus that they say are frustrating the public and adding to taxpayer costs. The White House has scrambled to fix technical issues and disputes Republican allegations that political motives were behind changes in the website's function. Tavenner's scheduled testimony before the House Ways and Means Committee is expected to offer insight into the decision-making. A key player, she was cleared to visit the White House 425 times between December 2009 and June 2013, including for several meetings with Obama himself, visitor logs show. One Oval Office meeting with Obama in March would have occurred as some technology officials in her agency publicly fretted about the possibility that the complicated website would malfunction, telling an insurance forum they were working to avert problems. Tavenner, 62, who was confirmed for her job by the Senate in May, was optimistic about the rollout when questioned by skeptical Republican senators at an April hearing. Tavenner is expected to be a critical witness this week because "she's more responsible for decisions made at CMS that probably led to this disaster," said Joe Antos, a healthcare analyst with the conservative American Enterprise Institute think tank. A committee aide, who spoke on condition of anonymity, said: "We expect her to be forthcoming. We think she'll be a very serious witness, and she's certainly integral. Tavenner appears one day before her boss, U.S. Health and Human Services Secretary Kathleen Sebelius, is due to testify before another panel in the Republican-controlled chamber. Committee aides hope that Tavenner can describe system problems at the more complicated back end of the federal marketplace, where consumers determine their eligibility for premium subsidies and enroll in coverage. Aides and experts fear new crippling problems could emerge as enrollment picks up in November and early December. There is also intense interest in Washington in learning who decided at the last minute to deny visitors to Healthcare.gov the ability to browse insurance plans without first creating a website account. That decision is widely blamed for the bottlenecks that helped paralyze the system as millions of visitors flooded the marketplace in the first days of enrollment and during the ensuing weeks. That (decision) had to be made at the highest possible levels, meaning in my view the White House. That's a strategic call about selling the reform," Antos said. White House visitor logs, which provide a public record of who visits with administration officials, have not yet been released for the August period when potential problems with the website launch may have been discussed. Republicans also want to know who in the administration decided to make Tavenner's agency the "quarterback" or system integrator for the huge information technology system behind Healthcare.gov. Analysts say that decision - rather than giving the job to the private sector - also may have created problems. Last week, the administration announced that it was handing the job over to a private contractor as part of the effort to fix the online enrollment system. CMS, the agency that oversees the massive federal Medicare and Medicaid programs, already had plenty to do before it took charge of implementing Obamacare, the Senate's leading Republican Mitch McConnell said in May, after voting against Tavenner's confirmation. Tavenner, who had served as acting administrator for more than a year, was nonetheless easily confirmed by the Senate on a 91-7 vote. Promising to run the agency like a business, she won accolades from leading Republicans who looked favorably on her career as a nurse and later as an executive for Hospital Corporation of America. She left HCA after 25 years to become Virginia's health and human resources secretary. Republican House Majority Leader Eric Cantor, a fellow Virginian, introduced Tavenner at her Senate hearing. He said he differed with Obama's healthcare policy, "but if there is anyone that I trust to try and navigate the challenges, it is Marilyn Tavenner." House Republican lawmakers at Tuesday's hearing are expected to focus not just on the healthcare website, but on the Affordable Care Act and its impact, aides said. "The website is terrible ... but the real problem is the law, which is causing people to lose coverage that they already have," one Republican aide said. Democrats will ask Tavenner what steps the administration will take to fix the reported website problems, one House Democratic aide said. The Democrats may focus on positive experiences of some of the 700,000 people who have filled out applications as a first step toward enrollment, including some who have been denied insurance previously because of pre-existing conditions, the Democratic aide said. Nonetheless, Democrats view the hearing as a largely political event staged by Republicans as part of their continued criticism of Obamacare, he said. On Friday, aides to committee Republicans were reviewing what Tavenner said on the record to Congress about the healthcare website before it went live, and comparing that with the actual rollout. Additional reporting by Gabriel Debenedetti; Editing by Marilyn Thompson, Martin Howell and Mohammad Zargham