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Washington (CNN) -- A moment of truth has arrived for President Barack Obama's signature health care reforms with Saturday's self-imposed deadline to get the problem-plagued HealthCare.gov website to work properly for most users.

Obama and officials in charge of the website say the "vast majority" of people who visit it starting Saturday will have a much improved experience than the crashes, error messages and delays that users faced when it launched October 1.

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Another stumble now, after recent revelations of policy cancellations and premium increases for some under the reforms known as Obamacare, would further weaken public trust in the administration's ability to implement the 2010 Affordable Care Act intended to help millions of uninsured Americans get coverage.

Continued problems also would provide more ammunition for fierce attacks led by conservative Republicans seeking to dismantle a law they consider unworkable and the ultimate example of big government run amok.

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According to Jeff Zients, a former administration official brought in to lead efforts to fix the website, success would mean that 800,000 people can successfully visit HealthCare.gov each day, with up to 50,000 of them online at the same time.

He told reporters at a White House briefing on Tuesday that teams working 24/7 still were making improvements intended to boost capacity to handle an expected surge in visitors beginning on Saturday.

The site was taken down Friday night for overnight maintenance ahead of the deadline set by the Obama administration following the rollout debacle for most users to have a smooth experience on the website.

"What we are comfortable with is that the 800,000 consumer visits a day will handle the demand across the coming months," Zients said.

It better. Republicans already are declaring the website and the sweeping health law a failure.

While the website exposed a serious flaw in executing the program and was a political embarrassment, Obama's poll numbers have slid sharply over individual policy cancellations that disproved his repeated refrain when selling the program publicly that people could keep their coverage if they preferred.

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"We've been down this road before as officials looked us in the eye for months, pledging to the American people that everything was 'on track' and we now know with the chaos behind the scenes that it was 'on track' for disaster," said a statement Friday by GOP Rep. Fred Upton of Michigan, who chairs the House Energy and Commerce Committee. "What reason do we have to believe things will be different now?"

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For his part, Obama sounded confident of the eventual success of both the website and the reforms he championed and pushed through Congress with no Republican votes, telling ABC in an interview broadcast Friday that providing the security of health care to millions of Americans "is going to be a legacy I am extraordinarily proud of."

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Problems continue to plague the system, and technology experts have questioned if the fixes being deployed by hundreds of government workers, outside contractors and specialists can get it functioning smoothly this weekend.

Luke Chung, president of Virginia-based software developer FMS Inc., called the administration's prediction that HealthCare.gov would work at 80% capacity on or around November 30 an impractical threshold in the software world.

"I don't know how to build something that's only 80% complete," Chung told CNN. "I don't even understand how that works."

The White House briefing led by Zients provided previously unknown details about the extent of the efforts to resolve the website problems.

He described a frenetic operation involving hundreds of workers in and around Washington.

The top priority is to make sure consumers can use HealthCare.gov to enroll in health exchanges that serve as the marketplace for private insurers participating in Obamacare, Zients said.

He described twice-daily conference calls and a 24-hour open line to allow for constant interaction for scores of computer programmers working for a dozen contractors to interact day and night.

"Ruthless prioritization"

"The point of these calls is we don't have an hour or a minute to waste, and therefore we need to make sure that there is ruthless prioritization at all times as to what matters most, that there's real-time reaction if something unexpected happened, someone hit a roadblock on a fix," Zients said.

The "ruthless prioritization" Zients mentioned means that much of the system supporting the HealthCare.gov site -- the "back end" part for making payments and interacting with insurance companies -- still was being built.

Robert Zirkelbach, a spokesman for the insurance trade group America's Health Insurance Plans, said that "there is still a lot of work to be done to make sure that enrollments can be done and processed accurately."

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Zients said more than 300 computer "bugs" have been fixed so far in a continuing process.

"Inevitably, as more and more users use the system -- which they are every day -- we find some new ones," he added. "But for the most part, they're not nearly as significant as the original bugs."

At the same time, he warned that demand could exceed capacity at times, especially if people rush to the site starting Saturday to sign up.

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To deal with that, Zients said a new "customer-friendly queuing system" will notify visitors of time to come back at the front of the line.

However, it was unclear what information -- if any -- the administration will provide to demonstrate how the website performs.

Officials have sophisticated programs that monitor second-by-second use of HealthCare.gov, but they have limited information made public so far to one monthly summary of overall enrollment figures.

Delay for small businesses

Some problems have proved insurmountable. The administration announced Wednesday that the website will be unable to enroll small businesses online for another year.

Small businesses can enroll in other ways in the special small business system known as SHOP, but problems with HealthCare.gov have prevented their online enrollment so far.

It was the third delay of the SHOP Marketplace component of the website, this time until November 2014.

Republicans pounced on the news, with House Speaker John Boehner urging Obama to delay implementation of the entire law.

"This law has been an absolute disaster, leaving us to ask, 'What's next?' " the Ohio Republican said in a statement. "If the President won't repeal it, he should at least delay the entire law before it wreaks any more havoc on American families and small businesses as well as our economy."

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Website woes aside, a new CNN/ORC International poll released Wednesday showed a majority of Americans believe the current Obamacare problems can be solved, and the figures for overall support and opposition remain little changed from a month ago.

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Website crucial to success

A properly functioning HealthCare.gov is crucial to implementing the most vital provisions of the Affordable Care Act that require people to have health coverage.

In theory, the law would create large pools of younger, healthier participants whose premiums would help offset the cost of providing care for older policy holders who use the health care system more.

The program must convince younger people who might be less inclined to pay for coverage they don't think they need.

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The GOP opposition targets the entire health care law, not just the website woes, as the ultimate example of big government overreach.

Initial enrollment figures lower

Enrollment figures for the first month after the opening of the new insurance exchanges were much lower than initially expected. But some states are now reporting stronger numbers.

According to a CNN count based on available figures, a little more than 200,000 people have signed up for Obamacare -- either through the national system or networks set up in 14 states and the District of Columbia.

In addition, more than 370,000 have signed up for Medicaid under state programs expanded through the health care reforms, the CNN count shows.

The enrollment period runs until March 31, and officials have said the target for the first year was 7 million people. To ensure they have coverage starting on January 1, consumers must sign up by December 23, the administration recently announced.

Chung, the software expert, cited December 23 as the most significant deadline, noting that demand would be huge because people by nature wait until the last minute.

Success for the website would be determined by both the number of users as well as how long they are in the system, Chung said, comparing it to a highway on which 50,000 people traveling 60 mph is smooth traffic while the same number going 10 mph is a jam.

For its part, the administration advised people to shop on HealthCare.gov at low-traffic times, which it described as weekday mornings and evenings or on weekends.

"There are 23 shopping days in December (for coverage starting January 1, 2014). No need to rush," Health Secretary Kathleen Sebelius advised Friday on the Huffington Post website.

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CNN's Jim Acosta, Dana Davidsen, Chris Frates, Leigh Ann Caldwell, Lisa Desjardins, Paul Steinhauser and J. Byron Wolf contributed to this report