The Democratic primary candidate is sweeping Western states but is struggling to overcome Clinton's significant lead with the delegates who ultimately will decide the party's nomination.
It's a situation that's oddly reminiscent of race eight years ago, when a frustrated and fatigued Clinton found herself fighting - and failing - to close then-Illinois Sen. Barack Obama's early delegate lead.
Just as Clinton did in 2008, Sanders and his team are vowing to take their fight all the way to the party convention in July with an aggressive push for delegates in next month's contests in Wisconsin, New York and five northeastern states. They're ratcheting up focus on her weaknesses, particularly with independents and younger voters. And Sanders is casting himself as the most electable Democrat in the general election, an effort targeted at wooing superdelegates, the party insiders who play a big role in picking the nominee.
There's little question that Sanders has tapped into a powerful force within the Democratic party. He's still drawing tens of thousands to his rallies - attracting 17,300 in Seattle on Sunday - and has collected more than $140 million in donations. His fundraising shows no signs of slowing down: Sanders raised more than $4 million since his victories in Saturday's contests in Washington state, Hawaii and Alaska.
But his campaign has always had a fight ahead of it. And despite his wins in 15 contests that shot seems to have grown even longer in recent weeks.
Even top staffers acknowledge that the path forward isn't easy.
"We're going to have to win states by significant margins," said senior strategist Tad Devine. "It's not going to be a big lead."
Based on primaries and caucuses to date, Clinton has 1,243 delegates to Sanders' 975. Including superdelegates, Clinton has 1,712 delegates to Sanders' 1,004, leaving her shy of the 2,383 it takes to win.
According to an Associated Press analysis, Sanders needs to win 67 percent of the remaining delegates and uncommitted superdelegates through June to be able to clinch the Democratic nomination. So far he's only winning 37 percent.
The difficult math is, at least in part, a reflection of how Clinton learned from her 2008 mistakes. One of her first hires was Jeff Berman, Obama's delegate guru. Her campaign invested early in their delegate strategy, a tactic that seems to have paid off. Her current lead of 268 pledged delegates is nearly double the margin that Obama held over Clinton during the 2008 primary.
Clinton's campaign believes they can knock out Sanders by the end of next month, arguing they will have racked up enough delegates after the April 26 contests in five northeastern states to make it mathematically impossible for him to win.
"He's going to contest these states, we're going to contest these states, but the truth is that after April 26 there is just not enough real estate for Senator Sanders to contest the lead that we've built," said Joel Benenson, Clinton's senior strategist.
While Clinton aides say they have no plans to push Sanders out of the race, they're eager to look past the primary. In interviews and speeches last week, Clinton largely focused on Tuesday's deadly attacks in Brussels, casting GOP front-runner Donald Trump and Texas Sen. Ted Cruz as unqualified to deal with complicated international threats.
She kicked-off her Wisconsin campaign with a Monday speech in Madison, where she urged voters to consider what Trump might do to shape the Supreme Court.
Sanders team plans to compete hard in Wisconsin and New York, hoping to not only win delegates but hit Clinton with the psychological blow of losing the state she represented in the Senate.
"Bernie is a native son of Brooklyn," said campaign manager Jeff Weaver, who called the Vermont senator the "voice of New York."
They also plan to try and persuade uncommitted superdelegates to back Sanders, arguing that his higher approval ratings and ability to motivate Democratic voters pose toughest challenge to Trump.
"This isn't an academic exercise now whether Trump could be the nominee. He's likely to be the nominee," said Ben Tulchin, Sanders pollster. "Democrats cannot win if we don't have an energized base."
His challenge, again, comes down to the numbers. There are only 216 uncommitted superdelegates out of 714 total, according to an Associated Press analysis.
Sanders also argues that superdelegates in states that he won will feel pressure to support his bid - a claim Clinton supporters dispute.
Sen. Tammy Baldwin, D-Wis., said she faced similar questions in 2008 about her allegiances to Clinton as a superdelegate after then-Illinois Sen. Barack Obama won her home state's primary.
Baldwin, who again endorsed Clinton, said she will be voting for the former secretary of state in next month's primary and it would be "odd" for her to vote for someone as a super delegate and then not pledge support for that candidate in the convention.
Besides, she noted, super delegates did not make the difference in the 2008 primaries.
"I do not necessarily believe that this one will be any different in that regard," she said.
TM and © Copyright 2016 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2016 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)
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