President Obama denounced "irresponsible" Republican critics of his health care law Thursday, saying their threats could trigger a damaging government shutdown and a U.S. credit default in the weeks ahead.

"They are now threatening steps that would hurt our entire economy," Obama said during a health care speech in Largo, Md.

Some members of the Republican-run House have said they will not approve a new spending plan -- or an increase in the debt ceiling -- unless the health care law is defunded.

Democrats who run the Senate said they will never allow such a tactic, and so did Obama on Thursday.

"That's not going to happen as long as I'm president," Obama told supporters gathered at Prince George's Community College. "The Affordable Care Act is here to stay."

Obama spoke just five days before a pivotal date: Tuesday, Oct. 1, sees the opening of new health care exchanges that are key to financing the plan and fulfilling its goal of insuring nearly all Americans.

In his speech, Obama likened the new Health Insurance Marketplaces to large group plans that will include companies competing for business, lowering prices while maintaining quality.

Tuesday also marks the first day of the next fiscal year. If the White House and congressional Republicans are unable to agree on a new spending plan, parts of the government will shut down. Some GOP members said they will not support any spending plan unless it makes changes to what they call Obamacare.

Republican critics also say government subsidies and regulations on insurance companies will force higher premiums for many Americans, and those costs will rise even more if not enough people sign up for the exchanges.

Citing polls that show public opposition to the law, Senate Minority Leader Mitch McConnell, R-Ky., said Obama is trying to sell his plan to "a skeptical public. It must be frustrating for him that folks seem to be tuning out all the happy talk."

Some Republicans also say they will not back an increase in the $16.7 trillion debt ceiling unless the health care law is changed. On Thursday, House GOP leaders discussed an option to raise the debt ceiling only if the health care plan is delayed for a year.

Obama said the debt ceiling gives the government authority to borrow money to pay its bills; otherwise, the government will default. The Treasury Department reports that it will be unable to repay debt starting Oct. 17.

Congress is obligated to increase the debt ceiling without strings because the nation's "full faith and credit" is at stake, and "you don't mess with that," Obama said. He again vowed he would not negotiate on the issue.

In defending his health care plan in general, Obama said there has been "a lot of misinformation" that has generated "a lot of confusion." He said the law fixes a "broken health care system," and no longer allows insurance companies to discriminate against people with pre-existing conditions or illnesses.

Obama cited a recent report from the Department of Health and Human Services that said the average American will have 53 different plans to choose from and 95% of uninsured Americans will see their premiums cost much less than expected.

Like any new major program, "there are going to be some glitches as this thing unfolds," Obama said, but those can be fixed along the way.

Obama said many Republicans are working to try and make the plan fail, but are worried that it will succeed and make them look bad politically.

Congress passed the law and the Supreme Court upheld it, he said, and he will not surrender it.

"We are going to see it through," Obama said. "The Affordable Care Act is here."

David Jackson, USA TODAY