LARGO, Maryland (AP) — With just five days to go before Americans can begin signing up for health care under his signature law, President Barack Obama on Thursday ridiculed Republican opponents for "crazy" doomsday predictions of the impact and forecast that even those who didn't vote for him are going to enroll.

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 while mocking Republicans for trying to block its implementation. "The closer we get, the more desperate they get," Obama argued.

"The Republican party has just spun itself up around this issue," Obama said. "And the fact is the Republicans' biggest fear at this point is not that Affordable Care Act will fail. What they're worried about is it's going to succeed."

House Republicans are inserting provisions that undermine the health care law into a short-term spending measure needed to avoid a government shutdown on Oct. 1 and into legislation that would increase the government's borrowing ability, which the Treasury says will hit its limit in mid-October.

Senate Republican leader Mitch McConnell said in a speech on the Senate floor Thursday that Obama is trying to sell the law to a skeptical public.

"It must be frustrating for the president that folks seem to keep tuning out all the happy talk anyway," McConnell said. "This law is a mess. It needs to go. It's way past time to start over."

Obama won loud applause from a friendly audience at Prince George's Community College in the Washington suburbs when he vowed that he wouldn't let Republicans block the law. "We are going to see it through. The Affordable Care Act is here," Obama said.

The six-month enrollment period for the exchanges starts Tuesday, with consumers in most of the country able to comparison shop between plans online. The Obama administration needs millions of Americans — especially young, healthy people — to sign up in order to keep costs low for everyone.

The White House said Prince George's County, Md., has a high rate of uninsured, with about 16 percent of residents under 65 without insurance. Obama's audience was full of the young people he is targeting for enrollment.

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. Small businesses will not be able to enroll online starting Oct. 1 when new health insurance markets go live and will have to enroll by paper, and the Spanish-language version of its healthcare.gov website will be not be ready to handle enrollments for a few weeks. An estimated 10 million Latinos are eligible for coverage.

Three-and-a-half years after Obama signed the bill into law, his nearly hourlong speech showed he's still having to educate consumers about what will be available to them and convince them to sign up. He predicted success once people learn they can save money or get insurance for the first time.

"Even if you didn't vote for me, I'll bet you'll sign up for that health care plan," Obama said.

Obama said Republicans want "to shut this thing down before people find out that they like it."

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 Louisiana Rep. John Fleming, who said earlier this month that "Obamacare is the most dangerous piece of legislation ever passed in Congress." He cited Minnesota Rep. Michele Bachmann's appeal to colleagues on the House floor six months ago to "repeal this failure before it literally kills women, kills children, kills senior citizens."

And he quoted New Hampshire state Rep. Bill O'Brien'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.

"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.