Sanders is surging in polls, and a sweep of the first two contests in Iowa and New Hampshire appears within his grasp.

Anxiety appears to be building in the Clinton campaign, which has seen the race tighten.

Clinton's performance at the first Democratic debate in October was a cornerstone of Clinton's strong fall, which underlined her status as the clear frontrunner.

Coupled with a grueling but successful day of testimony before the House Benghazi panel in which Republicans failed to land a solid blow, the solid debate performance quieted doubts about the former secretary of state's campaign.