Sanders and Buttigieg clash, aiming for a two-person race

One way to avoid a long primary would be for Mr. Sanders or Mr. Buttigieg to fully unite either liberals or moderates behind their campaigns. Such unity seems unlikely: Mr. Sanders must contend with Ms. Warren on the left, while Mr. Buttigieg is a long way from clearing the race of moderates like Mr. Biden and Senator Amy Klobuchar of Minnesota, who has drawn fresh interest from many voters since a strong debate performance on Friday night…

The possibility that moderate voters could rally behind Mr. Buttigieg has seemed to pose a dire threat to Mr. Biden, whose weak finish in Iowa has sent at least some of his New Hampshire supporters fleeing to Mr. Buttigieg and Ms. Klobuchar.

Kevin Donohue, a retired federal employee who attended a rally for Ms. Klobuchar on the campus of Southern New Hampshire University, said he had not entirely made up his mind but was leaning strongly toward supporting her. Mr. Donohue, 63, said he was also tempted to vote for Mr. Buttigieg, but had definitively ruled out Mr. Biden after watching him in a televised town hall event last week.