* Tech adviser cites night-and-day improvement
* Says site can handle 50,000 simultaneous users, 800,000 per day
(Updates with administration progress report)
Dec 1 (Reuters) - Two months after the disastrous launch of a key component of President Barack Obama's healthcare law, administration officials on Sunday said they had achieved their goal of getting HealthCare.gov operating smoothly but warned the website will need more fixes.
Obama's specially appointed adviser Jeffrey Zients said a five-week emergency "tech surge" had doubled the capacity of the online health insurance portal while making it more responsive and less prone to errors.
"The bottom line: HealthCare.gov on Dec. 1 is night and day from where it was on Oct. 1," Zients told reporters a day after the administration's self-imposed Nov. 30 deadline for making the website operate properly for the vast majority of users.
"We've widened the system's on-ramp - it now has four lanes instead of one or two," he said. "We have a much more stable system that is reliably open for business."
The administration's key achievement was to increase site capacity to 50,000 simultaneous users, which would allow HealthCare.gov to handle a minimum of 800,000 users per day.
But Zients warned that peak traffic volumes could eclipse the new capacity, temporarily preventing visitors from completing online applications for subsidized health coverage. And officials acknowledge that the site is still unlikely to operate smoothly for some visitors, even when volumes are within its capabilities.
Overall, the Zients team's success could mark a new chapter for Obama's signature domestic policy if HealthCare.gov proves stable enough to handle millions of potential applications from uninsured Americans as well as others whose current insurance policies face cancellation at the end of the year.
But a repeat of the Oct 1 launch, when the site crashed as millions of visitors flooded in, could leave hundreds of thousands of people without coverage and deal a staggering political blow to Obama's legacy and the 2014 election prospects of congressional Democrats.
Republican critics of the healthcare reform law known as Obamacare were quick to dismiss the administration's achievement as overstated.
"Have they made some progress? Yes. They brought in some private sector folks to try to get the functionality up. It still doesn't function right," Representative Mike Rogers of Michigan said on NBC's "Meet the Press" program.
Democrats, who face a tough congressional midterm election battle in 2014 that could be complicated by Obamacare's rocky rollout, sounded a cautious note. Some nervous party members have called for a delay of the law's individual mandate that requires people to be enrolled in coverage by March 31 or pay a penalty.
"This is going to take some time before it's up and kicking and in full gear," Democratic Representative Chris Van Hollen said on "Meet the Press."
Asked if the mandate needs to be delayed, Van Hollen replied: "As of today, no ... let's see how this is working."
HealthCare.gov will face increasing pressure in the days and weeks to come as consumers who want benefits in time for the new year have a Dec. 23 deadline to sign up. The Obama administration had hoped to enroll about 7 million people in 2014 under the Patient Protection and Affordable Care Act. Many of those consumers are expected to qualify for subsidies.
To work, the program must get millions of young, healthy consumers to sign up by March 31. Their participation is key to keeping the program's costs in check.
"There's still plenty of time to reach people who are now uninsured and especially the young and healthy who are key to keeping the insurance market stable," said Larry Levitt, a senior vice president at the Kaiser Family Foundation.
(Additional reporting by Patrick Temple-West in Washington; Editing by Bill Trott)