- Bernie Sanders is leading Hillary Clinton in a New Hampshire poll and is drawing big crowds
- Julian Zelizer: Clinton needs to learn from Sanders' success and focus more on the grassroots
Julian Zelizer is a professor of history and public affairs at Princeton University and a New America fellow. He is the author of "The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society" and co-editor of a new book, "Medicare and Medicaid at 50: America's Entitlement Programs in the Age of Affordable Care." The opinions expressed in this commentary are solely those of the author.
(CNN)Hillary Clinton can't afford to ignore Bernie Sanders any longer. She has a serious problem on her hands. Sanders is showing that his campaign poses a genuine threat. He is drawing massive crowds months before the caucuses and primaries begin and without much of a staff to speak of.