Speaking to an audience of about 800 at Boston's historic Faneuil Hall, the president said he took "full responsibility for making sure" the website is fixed as soon as possible, but insisted Americans are already better off under the ACA because it "requires insurance companies to abide by some of the strongest consumer protections this country has ever known -- a true patients' bill of rights."
He cited requirements to provide free preventive care, to cover people with pre-existing conditions, to forbid dropping coverage when people get sick and to abolish lifetime limits or restricted annual limits on benefits.
"All of this is in place right now," Obama said. "It is working right now."
The president noted the ACA -- commonly known as Obamacare -- was based on legislation enacted in Massachusetts when 2012 Republican presidential nominee Mitt Romney was governor.
"Mitt Romney and I ran a long and spirited campaign but I've always believed that when he was governor here in Massachusetts he did the right thing on healthcare," the president said.
"That doesn't mean it was perfect right away," he said, noting enrollment was slow initially but there is now "nearly universal coverage in Massachusetts and the vast majority of citizens are happy with their coverage."
The president said "there has been a lot of misinformation" about consumers receiving cancellation notices.
He said the ACA requires insurance companies to replace "substandard plans" with quality coverage and contended stories about cancellations are being exploited for political advantage by "the same people who have been trying to sink the Affordable Care Act from the beginning."
"If you're getting one of these letters just shop around in the marketplace," he said. "That's what it's for."
The president's speech was interrupted twice by protesters demanding he not approve plans for the Keystone XL pipeline.
"That is the wrong rally," he said after the first interruption. "We had the climate change rally back in the summer. This is the healthcare rally."
White House spokesman Josh Earnest told reporters aboard Air Force One, as the president traveled to Boston, Obama has "complete confidence" in Health and Human Services Secretary Kathleen Sebelius -- who testified before the U.S. Energy and Commerce Committee for 3 1/2 hours Wednesday on problems with the healthcare.gov website.
Earnest said Sebelius "took responsibility for many of the problems that are evident with the website, but she also deserves credit for the other aspects of the Affordable Care Act implementation that have gone well."
"The president has complete confidence in Secretary Sebelius," Earnest said. "She has been responsible, as she pointed out, for the broader implementation of the Affordable Care Act. "
"The administration has acknowledged that the Affordable Care Act implementation process is not going as smoothly as we would have liked, but we are confident that when we get to the end of this enrollment period on March 31 ... this will have significantly improved."
Asked about concerns personal information given on the website might not be secure, Earnest said people can be "confident" their personal information is secure.
Sibelius assured the committee site visitors would have a smoother time after the fixes are implemented by the end of November, and apologized to the American public for the troubled federal health insurance website rollout, saying the buck stopped with her.
"You deserve better. I apologize," Sebelius said. "I'm accountable to you to fixing these problems and am committed to earning your confidence back."