WASHINGTON — Contractors who built the web portal for the Obama administration’s health insurance marketplace said Thursday the site’s crippling problems trace back to insufficient testing and changes that government officials made just prior to going live.

Who’s to blame? The first congressional hearing into what went wrong dug into issues of website architecture and testing protocols — but also re-stoked the partisan battle over President Barack Obama’s signature expansion of health coverage for millions of uninsured Americans. Republicans who’ve been trying to kill the program the past three years sounded outraged that it is being poorly carried out, while Democrats jeered them as political hypocrites.

What was clear after more than four hours of testimony was that the contractors had only partial answers, and only the Obama administration can eventually put the entire picture together to explain the botched rollout.

Better times are coming, said executives from CGI Federal, which built the HealthCare.gov website serving 36 states, and from QSSI, which created a component that helps verify applicants’ incomes and other personal details. They said problems are being fixed daily and expressed optimism that anybody who wants coverage will able to get it by Jan. 1.

“The system is working, people are enrolling,” said CGI vice president Cheryl Campbell. “But people will be able to enroll at a faster pace.”

Asked for a timetable, she side-stepped, saying: “I don’t like to raise expectations.”

The online insurance markets were meant to be the portal to coverage for people who don’t get health benefits on the job. Middle-class people are to pick from subsidized private insurance plans, while low-income people are steered to Medicaid in states that have agreed to expand that safety-net program. But the administration is now urging consumers to apply via call centers or on paper forms as the website problems are being addressed.

Lack of testing was the main thread emerging from Thursday’s hearing before the House Energy and Commerce Committee.

But questions were also raised about a decision by the administration to not allow window shopping, as e-commerce sites generally do. Requiring consumers to open accounts and calculate subsidies before they could shop greatly increased the volume of traffic. That precipitated the crash of an accounts registration feature that became an early bottleneck. The site is now allowing limited window shopping.

“It’s certainly not perfect but getting better by the day,” Health and Human Services Secretary Kathleen Sebelius said in Phoenix.

“We now have 700,000 applications that have been submitted for health insurance. We intend to make sure those folks get the coverage they need,” she said.

At Thursday’s House hearing, the contractors said they each tested their own components independently but that the Health and Human Services Department was responsible for testing the whole system from end to end. That kind of testing didn’t happen until the last couple of weeks before the system’s Oct. 1 launch.

Representing QSSI, Andrew Slavitt told the committee that ideally, end-to-end testing should have occurred well before that, with enough time to correct flaws.

How much time?

“Months would be nice,” said Slavitt.

“We would have loved to have months,” concurred CGI’s Campbell.

The administration has acknowledged it did not test enough, and that that contributed to the problems.

The focus on the contractors is just a first step for re-energized GOP investigators. After the failure of their drive to defund “Obamacare” by shutting down the government, Republicans have been handed a new opportunity by the administration in the signup problems. Administration officials, including Sebelius, are to testify next week.

“This is not about blame,” committee Chairman Fred Upton, R-Mich., said as he opened the hearing. But he and other Republicans wasted little time in castigating the administration for having repeatedly assured Congress before the launch that everything was on track.

“Are they simply incompetent, or were they lying to the American people?” said Rep. Joe Pitts, R-Pa.

Democrats shot back that Republicans have no objectivity when it comes to the law. “We have already documented a record of Republicans trying to sabotage the Affordable Care Act,” said ranking Democrat Henry Waxman of California, citing not only the defunding effort in Congress but state-level campaigns to discourage enrollment.

Another Democrat, New Jersey’s Frank Pallone, dismissed the hearing as a “monkey court.”

What motivated the administration to delay the window shopping feature remained an unresolved question. QSSI’s Slavitt testified it came as a “late decision.”

Speaking for the administration, Julie Bataille, director of Medicare’s office of communications, told reporters Thursday without elaboration that it was a “business decision.” The Medicare agency is responsible for running the health care overhaul.

E-commerce sites, including Medicare.gov, routinely allow anonymous shopping, and customers set up accounts when they check out. Health and Human Services spokeswoman Joanne Peters said recently that window shopping wouldn’t have let consumers first see if they were eligible for tax credits. The credits amount to a discount off the sticker price of premiums.

Without citing any evidence, some Republicans suggested the administration’s motivation was political. “This browsable website was turned off to hide the costs,” said Rep. Cory Gardner, R-Colo.

Rep. G.K. Butterfield, D-N.C., sought to debunk that.

“Are you aware of any political intervention by this White House relating to your work on HealthCare.gov?” he asked Campbell.

“I am not,” she responded.

QSSI’s Slavitt said the decision had technical implications. It increased the website’s workload, contributing to the failure of an accounts registration function that his company was in charge of. Requiring the merely curious to create accounts “may have driven higher simultaneous usage of the registration system that wouldn’t have occurred if consumers could window-shop anonymously,” he said.

Slavitt added that accounts registration snags are being cleared up. And HHS has since incorporated a rudimentary window shopping feature to HealthCare.gov

Meanwhile, Slavitt and the administration both are saying that another, even more important component designed by QSSI is working well. It’s the website’s virtual back room, known as the federal data hub, which plays a crucial role in verifying applicants’ identity, immigration status and income.