Major contractors hooking up the internal plumbing of President Barack Obama’s health care law projected confidence Tuesday that they will be ready to go by an Oct. 1 deadline, even though the system is still being tested.

With just three weeks to go before new state health insurance markets launch, efforts are continuing to reliably link up government agencies, the markets themselves and private health plans.

The congressional Government Accountability Office warned this summer that with so much work left for the last minute, a timely and smooth launch could not be guaranteed. Still, with the president’s image at stake, his administration is working all out to deliver.

It got a boost this week when California officials said their marketplace — the nation’s largest — would start enrolling uninsured people Oct. 1 as scheduled.

Representatives of four big contractors testified in a rare joint appearance Tuesday before the House Energy and Commerce committee. Republicans who run the panel billed the hearing as a “pulse check” on Obama’s law.

Despite the contractors’ upbeat reports, a witness from a consulting firm advising states on the system said it still expects a difficult debut.

The insurance marketplaces that launch Oct. 1 are supposed to be the gateway to the new system. People can apply online, through a call center, in person, or on paper.

Coverage takes effect Jan 1. At that time, insurers will be forbidden from turning away people in poor health and virtually all Americans will be required to carry health insurance or face fines. Two recent independent studies of rates already filed by insurers indicate plenty of low-premium plans will be available, but that consumers who choose them will face high deductibles and copayments.

The four contractors each said they would be ready and that have strong safeguards in place to protect the privacy of personal data — an issue Republicans have lately raised as a potential vulnerability.

Cheryl Campbell, a senior vice president of CGI Federal Inc., which is building the marketplace the federal government will run in the 35 states that chose not to establish their own exchanges, said her company has met all the key milestones under its contract, including an operational readiness review this month.

Michael Finkel, an executive vice president of QSSI Inc., said he expects lynchpin of the system — a federal data hub that will link with federal agencies such as Social Security, Homeland Security and the Internal Revenue Service to verify identity, legal residence and income — to be ready as planned, although testing is still going on.

John Lau, program director for Serco, said his company is staffing up to handle paper applications and other tasks. It will open a service center in Kentucky on Oct. 1, followed by two others in Arkansas and Missouri.

Lynn Spellecy, a corporate lawyer for Equifax Workforce Solutions, said the subsidiary of the well-known credit reporting company is ready to help verify income and employment. Final testing is scheduled for Sunday.

But Brett Graham of the consulting firm Leavitt Partners, which is advising some states, said the markets can still expect a rocky debut. He cited the system’s massive complexity, the need for timely and accurate verification of personal details for millions of people and potential information security risks.