Transcript for 'Sometimes going negative is a necessary evil': Nate Silver
As Democrats we can't launch dishonest attacks against fellow Democrats. We have to beat Donald Trump. Now, Bernie campaign has unleashed a barrage of negative attacks on Joe Biden. Accused Joe Biden of supporting Paul's cut on social security. Bernie's campaign is not telling the truth. When I argued that we should freeze federal spending. We got some bad news for them, we're not going to cut social security. We're going to expand benefits. There you see dueling ads from Joe Biden and Bernie Sanders on social security in this final stretch to the Iowa caucus. History shows that Iowa voters have punished candidates that have gotten into public fights. We have Nate silver, if campaigns are making a mistake by going negative. Candidates historically are reluctant to go negative in the primaries. Especially early in a race. We're talking about attacking a candidate in your own party. But look a little deeper, after attacking Biden in the first debate kamala Harris surged to 14% in the polls and she faded after that. Pete buttigieg began rising in the polls going from 5% to 10% after attacking Elizabeth Warren on medicare for all. Warren fell from 24% to 15%. Not surprising to see Sanders and Biden clash, they have some big philosophical differences. But Warren and Sanders have a lot in common. They've been going after one another, too. More reason to attack each other and not less. Of courses there are risks. You can cite the example of dick Gephardt and Howard Dean in 2004. While John kerry surged into first place. There's less risk if you're punching up. Warren in third place nationally can stand to take some chances. She currently has 13% of winning a majority of our delegates in our forecast. Not helpless but not great. If she wins Iowa, her chance will shoot up to more than 50%. While Sanders might prefer to avoid a fight with Warren, his biggest problem is Biden in first place. So, no, I don't buy that going negative is a bad idea. Especially when you're trailing and Iowa is right around the corner.
This transcript has been automatically generated and may not be 100% accurate.