The morning after he lost the Nevada caucuses in February, Bernie Sanders held a painful conference call with his top advisers.

Sanders expressed deep frustration that he had not built a stronger political operation in the state, and then turned to the worrisome situation at hand.

His strategy for capturing the Democratic presidential nomination was based on sweeping all three early voting states, and he had fallen short, winning only his neighboring state New Hampshire — to the consternation of his wife, Jane, who questioned whether he should have campaigned more in 2015.

Without that sweep, his aides thought at the time, Sanders had little hope of overcoming his vast problems with black voters in the Southern primaries. And he had no convincing evidence to challenge Hillary Clinton’s electability.

“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.”

Sanders is now campaigning more effectively than many expected, exposing Clinton’s weaknesses as a candidate and positioning himself to win contests like the Wisconsin primary on Tuesday. But allies and advisers of Sanders say they missed opportunities to run an aggressive political operation in 2015 that would have presented more of a challenge to Clinton. She has now firmly built a big lead in delegates needed to clinch the nomination — a margin that would be smaller if Sanders had run differently last year, according to interviews with more than 15 people who are on his team or close to him.

Despite the urging of some advisers, Sanders refused last fall and early winter to criticize Clinton over her $675,000 in speaking fees from Goldman Sachs, an issue that he now targets almost daily. He also gave her a pass on her use of private email as secretary of state, even though some allies wanted him to exploit it. And he insisted on devoting time to his job as a senator from Vermont last year rather than matching Clinton’s all-out effort to capture the nomination. Some advisers now say that if he had campaigned more in Iowa, he might have avoided his critical loss there.

All those decisions stemmed in part from Sanders’ outlook on the race. He was originally skeptical that he could beat Clinton, and his mission in 2015 was to spread his political message about a rigged America rather than do whatever it took to win the nomination. By the time he caught fire with voters this winter and personally began to believe he could defeat Clinton, she was already on her way to building an all but insurmountable delegate lead.

The Democratic race could yet take unpredictable turns, and his campaign is vowing to win. If Sanders beats Clinton in her adopted home state of New York on April 19, he could raise damaging questions about her candidacy and gain more momentum. But he would still need to win a landslide victory there and in Pennsylvania, California and other states to overtake her in delegates this spring. For Sanders, the missteps of 2015 and early 2016 — leading to his delegate deficit — are the greatest drag on his recent success.

Bob Kerrey, a former Nebraska governor and senator who ran for the Democratic nomination in 1992, said Sanders might be winning now if he had relentlessly pressured Clinton since last fall over her closed-door speeches to Wall Street banks, her role in the finances of Clinton Foundation programs, and other political vulnerabilities. Sanders did not raise the paid speech issue, after long resistance, until late January.

“Making the transcripts of the Goldman speeches public would have been devastating” to Clinton, Kerrey said. “When the GOP gets done telling the Clinton Global Initiative fundraising and expense story, Bernie supporters will wonder why he didn’t do the same.”

Part-time and Unknown

Competing aggressively against Clinton in 2015 was not part of the Sanders strategy when he announced his candidacy last April. Rather, in early campaign planning meetings at his home and office near Burlington, Vermont, Sanders made it clear that he was focused on bringing his liberal message to cities and towns across America while also fulfilling his duties in the Senate. Advisers said they warned him about the travel demands that a serious presidential bid would entail. They noted that Clinton, who had left the State Department, would be working around the clock to campaign, raise money, nail down endorsements and develop policy plans.

But Sanders did not intend to match her schedule. He never considered resigning his seat, advisers said, and he thought he could compete effectively by campaigning about three days a week while the Senate was in session and then making weeklong trips when Congress was on break. As a result, he had limited time to campaign in crucial states like South Carolina; he canceled a visit to Charleston in mid-June after the church shootings there, and he did not return to the city until late August.

“Would I have wanted him on the road like 24/7? Yeah, of course,” said Jeff Weaver, Sanders’ campaign manager, who noted that candidate visits were more powerful with voters than television advertisements or pro-Sanders surrogates. Referring to Sanders’ work as a senator, Weaver added, “He does have this other responsibility which he takes very seriously, and I certainly understand that.”

Jane Sanders, an influential adviser to her husband, suggested that he might be in a better position today if he had campaigned more in 2015.

“We didn’t run all over the nation last year,” Jane Sanders said. “We spent every week in the Senate; and every weekend, three or four days a week he would be running around the country.”

“It’s something that gives you pause,” she added.

In Iowa, Bernie Sanders campaigned slightly more than Clinton — but he also had far more work to do there, having started the campaign 40 points behind her in polls. He waited to run campaign advertisements there until late fall, preferring to save his resources instead.

Devine said the senator might have won Iowa’s Feb. 1 caucuses if he had spent a few more days wooing voters in the western and rural parts of the state. “In retrospect, it would have been better to have spent a little less time in New Hampshire and some more time in Iowa, but New Hampshire was our flank and he had to defend it,” Devine said.

The senator won New Hampshire by more than 20 percentage points.

Sanders also resisted pleas from his aides to do the kind of retail-style campaigning that Iowa voters like. He wanted to do more large rallies instead, even though many Iowans like politicians winning them over face-to-face.

“Bernie would say, ‘If I’m at a diner having a cup of coffee, I don’t want candidates coming up talking to me,’” Devine said.

For Sanders, a close outcome in Iowa was a significant achievement on one level, given how far ahead Clinton had been. But he was disheartened with the Feb. 20 Nevada caucuses, where he lost by five points. In an interview, he said he had strong support from voters there but — because of time, resources, or other factors — his campaign could not turn them out.

“In Nevada, we knew where our support was,” Sanders said. “We brought out significantly less than we should have, and that’s the difference.”

Since then, Sanders has been campaigning virtually full-time, rarely returning to the Senate, and has gone on to win 14 states — including an upset victory in Michigan — to Clinton’s 18 states.

Reaching Black Voters

The morning after the Nevada vote, Sanders pulled Devine away from church and Weaver from breakfast to talk about strategy. They agreed that Sanders would still compete for the South Carolina primary on Feb. 27, but he would shift his plans for the March 1 Super Tuesday contests. Instead of spending money on ads and ground operations to compete across the South, Sanders would all but give up on those states and would focus on winning states where he was more popular, like Colorado and Minnesota, which would at least give him some victories to claim.

The reason: Sanders and his advisers and allies knew that black voters would be decisive in those Southern contests, but he had been unable to make significant inroads with them. He had hoped to. At one meeting with advisers in December, he suggested campaigning hard in Alabama in January, but his team insisted that he focus on winning Iowa, New Hampshire and Nevada. Clinton went on to rout him in Alabama, as well as in South Carolina and other Southern states, running up huge margins in African-American areas.

These victories allowed her to compile a significant delegate lead, given that Democrats award delegates based on the candidates’ vote totals.

Cornel West, one of Sanders’ most visible African-American surrogates, said that he thought that Sanders could win the nomination but that the senator should have fought to be “well-known quicker and much earlier” among voters, especially blacks.

“He had to hit TV, radio and venues in black America much earlier,” West said. “So that’s part of the onus on the campaign to get out his policies and to get out his history — who he is, what he stood for, for 50 years.”

Resisting Attacks

At several points in 2015, Sanders rejected strategies that most serious presidential candidates would embrace against an opponent.

In October, as they gathered at a hotel outside Las Vegas to prepare for the first Democratic debate, Sanders’ advisers urged him to challenge Clinton over accepting $675,000 from Goldman Sachs for delivering three speeches, according to two Sanders advisers. They thought the speaking fees meshed with the senator’s message about Wall Street excess and a rigged America. But Sanders, hunched over a U-shaped conference table, rejected it as a personal attack on Clinton’s income — the sort of character assault he has long opposed. She has the right to make money, he offered.

Throughout the fall, his team urged him to rethink that stand. The campaign’s internal polling and focus groups showed that Clinton was vulnerable on the issue. And she was hitting him, especially on gun control. Yet he refused.

Only in mid-January did Sanders change his mind, when news broke that Goldman Sachs had escaped harsh penalties for its role in the financial crisis. At the Jan. 17 debate, he challenged Clinton three times on the speaking fees. On Jan. 28, three days before the Iowa caucuses, he began running a tough ad on the subject.

By then, it was too late to make a big difference. He lost narrowly.

“If he had been tougher on her, it could have made such a difference in Iowa and the states that followed,” said Tom Fiegen, a former Iowa state senator who campaigned for Sanders.

Sanders is now putting forward tough arguments against Clinton — over donations from people who work in the oil and gas industry, for example. But he repeatedly passed up chances to make these cases at his well-attended rallies and three televised debates last year as well as in media interviews when asked about his opponent.

“The central complication with Bernie is that he never wanted to cross into the zone of personal attacks, because it would undercut his brand,” Devine said. “Is there another candidate who could have run a tough negative campaign against her from the beginning and been effective? Sure, but it couldn’t have been Bernie. That’s just not who he is.”