LARGO, Maryland (AP) — President Barack Obama on Thursday ridiculed Republican opponents for “crazy” doomsday predictions of the impact of his signature health care overhaul.

Obama gave a speech promoting his most important domestic achievement with just five days to go before Americans can begin signing up for health care under the law, a major social policy reform that aims to extend coverage to millions of uninsured citizens.

Obama’s tough words came as a Congressional impasse over the health care law is raising the threat of a government shutdown. Republicans, who control the House of Representatives, are trying to defund the health care program through a provision attached to a temporary spending measure needed by Oct. 1 to keep the government running. Democrats, who have a majority in the Senate, oppose the provision.

With polls showing many Americans still skeptical of the law known as “Obamacare,” the president went back to the basics of explaining how nearly 50 million uninsured Americans will be able to buy coverage in new government-run exchanges.

The speech showed that three years after the law was passed, Obama still has to educate consumers about what will be available to them and convince them to sign up. The Obama administration needs millions of Americans — especially young, healthy people — to sign up in order to keep costs low for everyone.

Obama mocked Republicans for trying to block the law’s implementation. “The closer we get, the more desperate they get,” he said.

“The Republican party has just spun itself up around this issue,” Obama said.

Republicans argue the health care law is hurting business and job-creation because of its taxes and other requirements. They especially oppose a mandate that requires all Americans to have health insurance as an intrusion into private-decision making.

“This law is a mess. It needs to go. It’s way past time to start over,” said Senate Republican leader Mitch McConnell said in a speech on the Senate floor Thursday.

The six-month enrollment period for the exchanges starts Tuesday, with consumers in most of the country able to comparison shop between plans online.

Obama acknowledged there would be glitches in getting the exchanges up and running, and even as he was speaking, administration officials were quietly telling key interest groups to expect initial problems signing up online for coverage.

Obama didn’t call out any of his Republican opponents by name, but he laughingly taunted some of their arguments.

He mentioned House Speaker John Boehner’s prediction right before the bill was signed into law in March 2010 that “Armageddon” was impending. He quoted a Louisiana congressman who said earlier this month that “Obamacare is the most dangerous piece of legislation ever passed in Congress.” He cited Rep. Michele Bachmann, a former Republican presidential candidate, as saying six months ago that the law must be repealed “before it literally kills women, kills children, kills senior citizens.”

And he quoted a New Hampshire state representative’s declaration in August that Obamacare is “a law as destructive to personal and individual liberty as the Fugitive Slave Act of 1850.” That was met by a chorus of gasps and boos from the largely black audience at a community college in the Washington suburbs.

“Think about that. Affordable Health Care is worse than a law that lets slave owners get their runaway slaves back,” Obama said. “I mean, these are quotes. I’m not making this stuff up.”

“All this would be funny if it wasn’t so crazy,” Obama said.

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