Neurosurgeon Ben Carson said Friday that ObamaCare is the worst thing to happen to the United States since slavery.

"I have to tell you ObamaCare is, really I think, the worst thing that's happened in this nation since slavery," Carson said during a speech at the Values Voter Summit of religious conservative activists.

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He argued that the healthcare law, like slavery, makes citizens subservient to the government, arguing the law is about control.

Carson emerged as a national figure in the conservative movement earlier this year after he gave a speech at the National Prayer Breakfast in which he criticized President Obama while Obama sat in the audience. Carson briefly opened the door for a national bid for office.

Later in April, Carson had to withdraw his name as commencement speaker at the Johns Hopkins University School of Medicine under pressure after controversial comments he made about gay marriage.

During a speech in September, Obama defended his healthcare law against a state lawmaker who had compared it to the Fugitive Slave Act.

Obama said his detractors became more desperate in their rhetoric the closer the deadline came to implementation.