Transcript for Jimmy Kimmel on son's health care battle, Trump
prank king. Known for candy-stealing. You ate it! Reporter: And "Mean tweets." You sound like a whining baby, does baby Jimmy want his bottle? I have a story to tell about something that happened to our family last week -- Reporter: Once reluctant to politicize his show, Jimmy Kimmel has become America's unlikely health care crusader following the birth of his son Billy six months ago, born with a rare heart condition. Before 2014, if you were born with congenital heart disease like my son was, there's a good chance you'd never be able to get health insurance because you had a pre-existing condition. Reporter: An emotional monologue that instantly went viral, putting a face on a health care debate. No parent should ever have to decide if they can afford to save their child's life. Was there a moment you decided I'm going to take something very personal and very real, and I'm going to try and change the conversation right now? I knew I had to say something. And it just so happened that at that time, this health care debate was going on. I looked around this hospital. It occurred to me that people need health care. In this case I thought, all right, what can I do or say that might potentially make some good out of this? Maybe make me feel like this happened for a reason? Reporter: Within days his name was invoked by senator bill Cassidy. Does it pass the Jimmy Kimmel test? Would a child born with a congenital heart disease be able to get everything she or he would need in that first year of life? Reporter: But when senator Cassidy cosponsored legislation a few months later, Kimmel was not impressed. And this new bill actually does pass the Jimmy Kimmel test, but a different Jimmy Kimmel test. With this one your xhooild child with the pre-existing condition will get the care he needs if and only if his father is Jimmy Kimmel. I don't think Mr. Kimmel did the country a good service by calling a good man a liar without at least talking to him. I get it, I don't understand because I'm a talk show host, right? Reporter: A back and forth between Republican lawmakers and kimmcoupkimmel mirroring the debate in congress. Do you believe every American, regardless of income, should be able to get regular checkups, maternity care, et cetera, all of those things that people who have health care get and need? Yep. So "Yep" is Washington for "Nope," I guess. You directly take on senator Cassidy, call him a liar. You are not afraid to call out politicians -- I believed bill Cassidy when he was on my show. Seemed like good man, maybe he is a good man. Just because he's a doctor doesn't mean he understands how health insurance works. I do give him the benefit of the doubt, but all I know is he told me one thing and what was brought to the floor was entirely different. Reporter: And then last week as Americans were reeling from the deadliest mass shooting in modern U.S. History, Kimmel's emotional monologue reflective of a nation in mourning and disbelief. There's probably no way to ever know why a human being would -- do something like this. To other human beings who are at a concert. Having fun and listening to music. Reporter: And just as he did with health care, Kimmel called out politicians to take action. 89% of Republicans and Democrats are in favor of restricting gun ownership for the mentally ill. But not this gang. I just happened to be in a position two times this the year where something happened that really hit me very close to home. Once with my child, of course. Then literally close to home. Las Vegas. Reporter: Not everyone is a fan. Kimmel ped fun at his newfound political celebrity by reading his own mean tweets following his health caretakedown. I made a break through based on the thoughtful responses I got on Twitter and Facebook from people like Thomas W wrote, why don't you go somewhere else like a different country if you don't like our president, and stop your crying on TV, snowflake. What they're doing now is they're trying to drag up any kind -- especially these gun nuts, trying to drag any -- take any comedy bit I get out of context, use it as some kind of proof, they're saying that I'm calling myself the moral conscience of America, which I'm most certainly never did, most certainly never would. Reporter: Next week his show will be broadcast from Brooklyn, returning to his birthplace. Brooklyn is a magical place, it really is. Every time I bring my kids there I feel guilty that they don't get to just run around the neighborhood with a bunch of other kids. We don't really do that here in L.A. We don't really know our neighbors until we're being deposed by them. Reporter: With a slew of special guests, including late-night legend David Letterman. Yeah, this is the first time he's done a late-night show since his last show. I feel like I've been preparing questions for Dave for my whole life. Any chance you could get president trump there? I -- I would guess that he's not planning to visit me any time soon. I think he and Sean hannity have date night on those nights. It's tough to break away. Would you like to have Donald Trump on the show? Absolutely. I have a lot of questions for Donald Trump. What would be your first one? Well, I really -- I mean, maybe this is crazy, but I feel like I could turn him around on a couple of things. Reporter: Call it comedic persuasion, Kimmel hopes his outspokenness might change more minds than just the president's. Would you like the show to have an impact more than just making people laugh? Of course we would love to make an impact in a positive way. That said, I have no delusions of grandeur. My hope is people know me well enough to know I'm coming from a good place and they can agree with me O disagree with me, whichever they like. But I appreciate the opportunity to share my opinions with them. What I'm talking about tonight isn't about gun control, it's about common sense there are doctor "Nightline" I'm Amy robach in Los Angeles.
This transcript has been automatically generated and may not be 100% accurate.