Transcript for Bernie Sanders: Behind the Scenes on the Campaign Trail

Some unexpected twists for republicans in the already unconventional primary season. For democrats the rising tide for the outsider candidate. Will this wave of support be enough for Bernie sanders to win South Carolina tomorrow? Our David Wright spent one-on-one time today probing for secrets from sanders. Reporter: Orange berg bark. Home of the panthers and the lady panthers. For one night only, senator Bernie sanders. You have the right to vote, end of discussion. Reporter: We're with him backstage. How do you keep your energy level up? Bus I'm young, I'm not old like you. Running all over the country. We're doing great. These kinds of events really energize me, tell you the truth. Reporter: Down to the wire in the 2016 race for the white house. The primary season quickly coming to a crescendo. Especially over on the republican side. Marco Rubio is a lightweight. He's not a strong guy. Reporter: Perhaps the biggest development today, a major endorsement. The best person to beat Hillary Clinton in November on that stage last night is undoubtedly Donald Trump. And so I am thrilled to be here this morning to lend my support. Reporter: New Jersey governor Chris Christie endorsed Donald Trump. He's rewriting the playbook of American politics because he's providing strong leadership that's not dependent upon the status quo. Reporter: That after a bruising performance last night at the tenth gop debate. Donald, relax. I'm relaxed. You're the basket case. Go ahead. Donald -- Don't get nervous. Marco -- Go ahead. I promise you -- Reporter: Ted Cruz and Marco Rubio tag teamed Donald Trump. Last-ditch effort to stop him. Gentlemen, gentlemen -- Reporter: Today at their separate campaign events, the schoolyard brawl continued. With Rubio calling trump a con artist. It's time to pull his mask off so that people can see what we are dealing with here. Reporter: Trump calling Rubio a choke artist. Once a choker, always a choker. And that was one of the epic meltdowns. He didn't know where he was. I thought he was going to die. Reporter: Today Donald Trump mocked that moment during Rubio's state of the union response to the president's state of the union. It's Rubio! Reporter: Today trump splashed water around as though it were flop sweat from Rubio. For his part, Rubio suggested trump may have Peed his pants during last night's debate. During two of the breaks he went backstage, he was having a meltdown. Then he asked for a full-length mirror. I don't know, why the podium goes up to here. He wanted a full-length mirror. Maybe to make sure his pants weren't wet, I don't know. Reporter: Compared to that -- Tearing people down is easy. Building them up is what we need to be doing together. Reporter: The democratic side seems downright tame. It feels like this time next week we could know who the nominees are. I think probably not, actually. Let's assume you run the table and you're the democratic nominee. Trump's the republican nominee -- That would be a lot of fun. How do you take him on? A man whose ideas and posture and behavior so is divisive, which really becomes an embarrassment to the united States of America. I don't think that's going to be a tough race. Reporter: Both of them are drawing huge crowds, including many first-time voters. Both rail against a political system that they say is bought and sold by special interests. We the people are going to have a government that represents all of us, not just a handful of billionaires. Reporter: One rails against billionaires. The other insists his billions are what allow him to be his own man. I know the best people and I'm totally self-funding my campaign. In an odd way you guys are flip sides of the same coin. No, I don't think so. Talking about a broken system. Tapping into people's frustration -- No, one of us is tapping into hatred. One of us is tapping into a very positive agenda. Imagining the day when you're on that stage with Donald Trump what do you say to him, what's your strategy for taking him down? Why would I tell you? He's probably watching your show. Reporter: This is sanders' penultimate stop in the palmetto state and truth be told it's a disappointing crowd. ABC's Mary Alice parks is traveling with the sanders campaign. This is probably one of the smallest Bernie sanders events I've been to. Is it your sense he's written South Carolina off? It's a state they'd rather have in the rear-view mirror. Reporter: Sanders is putting in his time, but mostly he's put his focus on the states voting on Tuesday and beyond. In orangeberg the two democrats almost ran into each other. The orangeburg fish fry and oyster bake, Hillary Clinton's inside right now. Bernie's waiting, he's on next. Inside -- fancy running into you here. Reporter: All the reporters traveling with the Clinton campaign. I've been to all the early states with her, this is Clinton country, people like her here. Reporter: In South Carolina, the democratic electorate is more than 50% black. We need you. Yes, I'll be there for you. Reporter: Bill and Hillary Clinton have been canvassing the state making 22 stops here this past week alone. Sanders has made a handful. Sanders has sought inroads in the black community. Wake up, South Carolina! Reporter: Including a new endorsement and radio ad from director spike lee. When Bernie gets in the white house, he will do the right thing. I'm Hillary Clinton and I approve this message. Reporter: Clinton has plenty of celebrity endorsements too, including two ads from one of the most celebrated voices in America. Actor Morgan Freeman -- She understands that our country can't reach its potential unless we all do. Reporter: Last week voters at this barbershop told us they'll make up their own minds. That's just like politicians go to our churches when it's election time but we don't see them no other time. Again, you're going to have the uneducated that's going to fall for it. Those of us who pay attention, that's not working. That actually salts us, makes us not want to be in your camp. Are you only saying that because you want our vote? Or are you really sincere? The real question is will we ever really know? Reporter: Clinton and sanders have almost the same number of pledge delegates. How are you? Reporter: It looks like she's about to get a big boost here in South Carolina. Clinton also has a massive lead among so-called superdelegates. Party grandees who get a say at the convention regardless of the outcomes of the primaries. The political establishment supports Hillary Clinton, end of discussion. If we do well and we start winning a whole lot of states and if the polls continue to show that I am a stronger candidate, I think a lot of these superdelegates think, maybe I'm not crazy about Bernie sanders, but what is most important to us is defeating the republican candidate. Donald Trump! Reporter: Barring a big surprise Tuesday night on the republican night, it's looking more and more likely that the gop candidate will be a certain brash New York billionaire. The republican race seems like it's beginning to wrap up. The democratic race, a lot of people are expecting it to as well. But Bernie sanders says he's not going anywhere, he's in this thing to the finish. He insists he sees a pathway to the democratic nomination. What we're trying to do is literally unprecedented. It will be one of the great political upsets in American history. Do I think we have a path to victory? I do. Are you in this all the way up to the convention? Absolutely, no, no. California obviously is 20% of the delegates from California, 20%. You're not going anywhere? We're going to California! We're off to the west coast, absolutely. Reporter: Win or lose big tomorrow, he's settling in for the long haul. I'm David Wright for "Nightline" in orangeburg, South Carolina.

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