“If Clinton had lost Iowa, New Hampshire and Nevada, it would have been a devastating series of defeats that would have called into question her entire campaign,” said Tad Devine, one of several Sanders advisers who described the Feb. 21 conference call. “We had to shift our strategy. But no matter what, the nomination became tougher to win.”
Mr. Sanders is now campaigning more effectively than many expected, exposing Mrs. Clinton’s weaknesses as a candidate, and is positioning himself to win contests like the Wisconsin primary on Tuesday. But allies and advisers of Mr. Sanders say they missed opportunities to run an aggressive political operation in 2015 that would have presented more of a challenge to Mrs. Clinton. She has now firmly built a big lead in delegates needed to clinch the nomination — a margin that would be smaller if Mr. Sanders had run differently last year, according to interviews with more than 15 people who are on his team or close to him.