He has broken with many Republicans on taxing the rich, threatening trade wars and keeping Planned Parenthood alive. On Friday, Donald J. Trump faced criticism for an even bolder act of conservative heresy: embracing the core tenet of the Affordable Care Act.

Mr. Trump has to date offered only bits and pieces of his health agenda, generally presenting a vow to repeal “Obamacare” and replace it with “something great.”

In a town-hall meeting hosted by CNN on Thursday night, he shared some more expansive views on the subject, and unlike most Republicans he did not call for removing the individual mandate that requires Americans to have health insurance.

Asked how people with pre-existing medical conditions would purchase insurance if the health law and the mandate were eliminated, Mr. Trump said, “I like the mandate.”