WASHINGTON (AP) — President Barack Obama on Monday offered "no excuses" — and little explanation — for the cascade of computer problems that are marring a key element in his health care law and giving his opponents fodder to criticize his signature legislative accomplishment.

The troublesome rollout of HealthCare.gov, the website where many Americans are supposed to be able to sign up to for insurance plans, has been a glaring embarrassment for the Obama administration and could impact next year's congressional elections as well as the president's legacy.

The health care law is intended to extend health insurance coverage to millions of Americans who lack it. Republicans, who argue that taxes and requirements associated with the 3-year-old law are costing jobs, have repeatedly failed to thwart its implementation. Their latest attempt triggered a 16-day partial government shutdown that initially overshadowed the problems with the rollout of the website.

The computer issues have called into question whether the administration is capable of implementing the complex policy and why senior White House officials — including the president — appear to have been unaware of the scope of the problems when the exchange sites opened on Oct. 1.

Software developers tasked with building the site said they saw signs a year ago that the debut could fail.

One source of the troubles appears to be the testing procedures employed before the rollout three weeks ago. Several developers of the website told The Associated Press they were worried for months about the system's readiness and whether the software meant to link key computer systems was being properly put through its paces.

Obama insisted that the problems would be fixed and all Americans seeking insurance would be able to sign up. But it was not clear how quickly that would happen. The administration is beefing up call centers and encouraging more people to enroll over the phone while the website problems persist.

"There's no sugarcoating it," Obama said. "Nobody is more frustrated than I am."

The president acknowledged that the failures would provide new fodder for opponents of the law, often referred to as "Obamacare." With the website not working as intended, "that makes a lot of supporters nervous," he said.

But he said, "it's time for folks to stop rooting for its failure."

People have until March 31 to sign up for coverage. The non-partisan Congressional Budget Office had projected that about 7 million people would gain coverage through the exchanges during the first year.

The president on Monday guaranteed that everyone who wants to get insurance through the new health care exchanges will be able to, even if they have to enroll over the phone or fill out a paper application.

Obama, in his most extensive remarks about the health care problems, insisted Monday that the health care law is about more than just a website.

"The essence of the law, the health insurance that's available to people, is working just fine," he said during his 25-minute remarks.

Administration officials initially blamed a high volume of interest for the frozen computer screens that many people encountered when they first logged on to the website. Since then, they have also acknowledged issues with software and some elements of the system's design.

However, the White House has yet to fully detail exactly what went wrong with the online system.

It appears the problems were well-known to some of those designing the system. One developer said that in the weeks leading up to the Oct. 1 launch, he and his colleagues huddled in conference rooms trying to patch deficiencies in computer code.

"It was an extremely tight deadline," said the developer, who spoke on condition of anonymity because he was told not to talk to the news media about his work.

The White House says more than 19 million people have visited HealthCare.gov since the site went live on Oct. 1. Officials also say a half million people have applied for insurance on the federal- and state-run websites.

Administration officials have so far refused to say how many people have actually managed to enroll in insurance plans. Without enrollment numbers, it's impossible to know whether the program is on track to reach the projections from the Congressional Budget Office's target figure.

Uninsured Americans have until about mid-February to sign up for coverage if they are to meet the law's requirement that they be insured by the end of March. If they don't, they will face a tax penalty — one of the provisions in the law that Republicans vehemently oppose.

Officials say that at this point they are not considering extending the enrollment window beyond March 31.

But the White House appeared to open the door to not penalizing those whose efforts to sign up were confounded by the system's problems.

"The law is clear that if you do not have access to affordable health insurance, then you will not be asked to pay a penalty because you haven't purchased affordable health insurance," White House spokesman Jay Carney said.

Using an expansion of Medicaid, the government-funded program that provides health care coverage to the poor, and government subsidies to help cover the cost of private insurance premiums, the Obama administration would eventually like to cover at least half of the nearly 50 million Americans who are uninsured.

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Associated Press writers Julie Pace, Laurie Kellman and Ricardo Alonso-Zaldivar contributed to this report.

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Follow Julie Pace at http://twitter.com/jpaceDC