WASHINGTON — With the next time to buy health plans under the Affordable Care Act starting in less than a week, the Obama administration is expressing confidence that HealthCare.gov is no longer the rickety online insurance marketplace that exasperated consumers a year ago.

Behind the scenes, however, federal health officials and government contractors are scrambling, according to confidential documents and federal and outside experts familiar with this work. They have been making contingency plans in case the information technology or other aspects prove less sturdy than the administration predicts. And some preparations are coming down to the wire.

As the Nov. 15 beginning of open enrollment nears, administration officials have been eager to draw public attention to the fact that parts of the computer system have been rebuilt, online insurance applications will be easier to use and new federal managers are in charge. And on Sunday officials announced consumers will be able to explore benefits and prices of 2015 health plans starting Monday, five days before the sign-up period begins.

“We’re really making sure that that website works super well,” President Obama said at a news conference a few days ago. “We’re double- and triple-checking it.”

Despite such efforts, the confidential documents written in recent weeks hint at elaborate backup planning that undercuts the administration’s predictions that an improved HealthCare.gov will be able to handle everyone who wants to sign up.

More than 8 million people in the U.S. enrolled for this year, including 44,258 in Maine.

More broadly, the documents reflect the high stakes confronting the administration as it tries to avoid last year’s mistakes and deal with new threats to the Affordable Care Act: the Republican gains in the midterm elections and the Supreme Court’s decision to review the government insurance subsidies that are a linchpin of the law.

One document from late October, for instance, describes a new system known as “throttling,” which will be deployed if the number of people trying to use HealthCare.gov at the same time strains the website’s enlarged capacity.

This throttling could send groups of people using different parts of the site into separate online “waiting rooms.” It is a “structured approach,” says the document from the Department of Health and Human Services’ Center for Medicare and Medicaid Services, the agency overseeing HealthCare.gov. This method, the document says, would detect overloads sooner and be more efficient than the one long online queue federal officials devised last winter to deal with the frozen error messages – dubbed the “blue screen of death” – that many consumers got when the exchange opened last fall.

As the sign-up period approaches, consumer advocates say they are optimistic that HealthCare.gov will work much more smoothly than a year ago, but they are not sure. “Has enough been done to fix the problems we saw for open-enrollment one? . . . We’re all waiting with bated breath until November 15,” said Mara Youdelman, managing attorney of the National Health Law Program.

NOTICES DELAYED

Other confidential CMS documents show that federal health officials drafted contingency plans involving notices for people who have insurance through the exchanges. The notices provide important information, such as whether enrollees appear eligible for federal subsidies for the coming year.

The notices were supposed to be in consumers’ hands by Nov. 1. But by the third week of October, a document says, fewer than 1 million of 7.6 million notices were ready to be mailed or emailed. Officials considered sending some late to certain groups, sending skimpier notices or reducing the size of ones delivered electronically. In the end, technicians fixed a computer problem, and the notices are going out in batches that are due to arrive by Saturday, when enrollment begins.

Nearly 90 percent of Mainers enrolled in the federal marketplace this year received subsidies, according to data released by the U.S. Department of Health and Human Services in June.

Meanwhile, another October document says that insurers will continue to test a new part of HealthCare.gov for small businesses – not ready last year – for about a month after enrollment gets underway. Andy Slavitt, CMS principal deputy administrator, said that parts of the small-business exchange, known as SHOP, that must be ready by Nov. 15 will be checked enough by then, while testing will continue on other aspects, such as the system’s ability to relay certain premium information back to employers.

In another behind-the-scenes effort, testing continues to try to ensure that the computer system is safe from hackers. The Government Accountability Office recently joined congressional Republicans who oppose the health care law in warning that the system does not have strong enough privacy protections. Administration officials have said that the system is secure, but computer experts from the Department of Homeland Security as well as HHS are trying once a day to hack into HealthCare.gov to see if the system is vulnerable, a CMS spokesman confirmed.

Some details about the readiness of the website are unavailable partly because CMS officials this year have insisted that every insurer involved in testing the site sign a confidentiality agreement requiring them to refrain from discussing the results. Insurers’ “end-to-end” testing of the site began in early October – about five weeks before the start of enrollment, instead of 10 days ahead of time last year.

STRADDLING A LINE

For their part, top federal health officials are straddling a line, working to build public confidence without overpromising. The system “will be very, very good,” Slavitt said in an interview. “I won’t commit to perfection. . . . The one thing I can predict is we didn’t think of everything. . . . It will continue to improve along the way.”

HHS Secretary Sylvia Mathews Burwell has pointed out that HealthCare.gov will have two jobs this year: signing up new insurance customers while, for the first time, renewing coverage for existing ones.

The enrollment window will be half as long as the first one, which began in Oct. 1, 2013, and ran through the end of March. This time, open enrollment is scheduled from Nov. 15 to Feb. 15. People who want coverage to begin in January must sign up within the first month – by Dec. 15.

The administration has refrained from saying how many uninsured Americans it hopes to attract this time. The Congressional Budget Office has predicted that 13 million people will be covered in 2015 through health plans sold through the federal insurance exchange and separate ones run by one-third of the states. That would be nearly twice as many people as signed up for 2014.

To try to simplify an enrollment process widely criticized as cumbersome, HealthCare.gov will steer some people applying for insurance for the first time to a rebuilt part of HealthCare.gov known as “Application 2.0.” It contains 16 computer screens of information to fill out, compared with 76 screens in the original version, and is designed for new applicants without complicated life circumstances, such as being an immigrant or having children living in different states.

Consumers who want a second year’s coverage can choose to shop again for a health plan and find out whether they qualify for the same subsidy, as administration officials are encouraging. If they don’t, HealthCare.gov will renew them automatically.

“I’m feeling good about it,” said Frank Principi, executive director of the Prince William Community Health Center in Northern Virginia, which will provide help to consumers signing up.

– With staff reports