About 6.4 million people currently receive such subsidies to offset the cost of their insurance in 34 states served by that exchange. Analysts have predicted that if those subsidies are removed, up to 10 million people in those states would lose health coverage by 2016, and individual health insurance premiums would skyrocket.
Underscoring that threat, Obama noted how the ACA has been followed by a sharp reduction in the number of Americans without health insurance, "driving the uninsured rate to its lowest level ever."
About 16 million previously uninsured people—1 out of every 3—have gotten covered as a result of Obamacare.
Obama also noted how the ACA has barred insurers from denying coverage or charging higher prices for people with existing health conditions, how the law mandates that insurers cover a slew of medical services without out-of-pocket payments from customers and how women are no longer forced to pay more for their insurance than men.
"Tens of millions of people enjoy new protections with the coverage they have," he said. "They might not know they have a better deal now, but they do."
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He added that Medicare has been "strengthened and protected," and said the average insurance premiums paid by families are now $1,800 lower than they would have been without the ACA.
The president also cited examples of people who had benefited from Obamacare, such as a Texas family whose previously mute son was now able to speak the words "I love you" to his parents, and a Pennsylvania woman who had been wheelchair-bound and in "constant pain" for years due to untreated osteoporosis.
That woman had sent him a letter describing how she underwent knee replacement surgery after getting insurance.
"She's back on her feet, she walks her dog, she's cooking, exercising," Obama said.
"Is there any greater measure of life and liberty and the pursuit of happiness than in those simple pleasures that are afforded because you have good health and you have some security?" Obama asked.
The president scoffed at the "gloom and doom predictions" that Obamacare opponents made before the ACA was passed into law.
"We were told again and again that Obamacare would be a job killer," Obama said, adding that people "amazingly" still make that argument.
But in reality, he said, "America has experienced 63 straight months of job growth ... which started the month we passed the Affordable Care Act."
He added, referring to opponents who call for the repeal of the ACA, that, "It seems so cynical to want to take away care from millions of people ... to punish millions with higher costs of care."