President Obama on Monday defended himself against Republican criticism that he misled the public over his signature health care law, telling supporters that his administration could not in good faith allow people with inferior insurance plans to keep them.

Obama has come under heavy criticism for his promises before the law was passed in 2010 that Americans who were happy with the plans they purchased on the open market could keep them. Since then, millions of people have received notices from their insurers that their plans have been canceled because of the new federal requirements.

But at a health-care summit of supporters, Obama said the change was necessary because many of those insurance plans would not cover the costs of medical care.

"If we allowed these old plans to be downgraded ... when the law was already passed, we would have broken an even more important promise: making sure Americans get access to health care that doesn't leave them one illness away from financial ruin," the president told more than 200 members of Organizing for Action, the nonprofit group started by his former campaign apparatus to support the administration's agenda.

"The bottom line is we are making the insurance marketplace better for everybody," Obama added.

Many Americans who have had their plans canceled have said they will have to pay more to buy new insurance under the Affordable Care Act. But the president, striking a combative tone, said his critics were wrong to seize on these concerns as evidence that the health-care law had added new burdens to millions of people.

"One reason we took up health-care reform was to help the uninsured, but also to help the under-insured or the badly insured," Obama said. "We did this to put an end to individual markets that had almost no standards. Thousands of Americans were dropped from coverage every year. People are acting like this was some new phenomenon, but every year there was churn in this individual market; the average increase was double digits on premiums in this same market."

Obama's appearance was part of an effort to keep his most ardent supporters invigorated to continue talking up the benefits of the health care law and helping sign up the uninsured before the deadline next March. David Plouffe, Obama's former senior adviser, and Massachusetts Gov. Deval Patrick (D) also addressed the crowd, sharply attacking Republican lawmakers who have opposed Obamacare.

The president pledged to make sure the health-care Web site would be fixed by the end of November, and he encouraged supporters to focus on the success stories. He said he got a letter from a man who bought insurance plan that lowered his monthly payments by half, which the man said "breathed life back into our family."

"It's not new that we have a bunch of folks spreading fear, misinformation and cynicism about this law. We've been facing that for four years. We're used to that," Obama said. "I've run my last political campaign, but I've got one more campaign in me: The campaign to make sure that this law works for every single American."