Washington (CNN)Bernie Sanders has a problem: He isn't Elizabeth Warren.
Liberal Democrats have been trying to get Warren, the senior senator from Massachusetts, to run for president for the better part of a year. Those vocal activists want a liberal option to push Hillary Clinton, the prohibitive favorite for the nomination but seen by some liberals as too politically moderate for their support.
Sanders, who is heading to New Hampshire for his first official campaign appearances on Saturday, wants to be that liberal option. His core positions -- breaking up Wall Street banks, making public college free, investing billions on infrastructure and guaranteeing health care for all -- are the same issues liberal groups have been championing for years. And his early campaign is starting to court organizers in key presidential states.
But as he settles into the Democratic presidential race, the response from some liberal groups and organizers has been markedly more focused on Warren, the middle-class champion and former Harvard professor, not Sanders, the Brooklyn-born, independent lawmaker with a gruff personality and an affinity for the moniker "democratic socialist."
Anna Galland, executive director of MoveOn.org Civic Action, welcomed Sanders into the race and touted his record on Wednesday before noting that the Vermont-based Move On and their allies would "continue to call on Sen. Elizabeth Warren to also bring her tireless advocacy for middle-class and working Americans to the race."
Democracy for Action, another liberal group whose views track closely with Sanders, had a similar reaction.
Our "members are excited to have progressive champion Senator Bernie Sanders join the 2016 presidential race," said Charles Chamberlain, the group's executive director, before adding, "We continue to encourage Senator Elizabeth Warren to join the race for president."
Both Move On and Democracy for America have dumped millions into a campaign urging Warren to run. But with every interview the Warren does, that goal looks less likely.
"I am not running and I am not going to run," Warren bluntly said in March.
The dynamic of liberal groups lining up behind a candidate who says she isn't running quietly bothers some Sanders' aides. They look at the money groups are spending to draft Warren and can't help but think about what that money could do for them.
"Obviously, one would hope one would have as much support as possible from all walks of life," Sanders said earlier this year when asked why he thinks groups like Move On aren't rallying around him. "I am a great fan of Elizabeth, and as for what people do and why they don't do it, I am not going to speculate."
Tad Devine, one of Sanders' top campaign advisers, said Thursday that he isn't particularly bothered by the clamoring for Warren because "she isn't going to run for president."
"There are a lot of people out there who are looking for someone like Elizabeth Warren and are really interested in those issues," Devine said. "I think when Bernie starts talking about those issues in his own way, it is going to be a lot easier for someone like him to get their support, than someone like Clinton, O'Malley, Webb or Link Chaffee."
Devine added: "I view those people as tremendous targets of opportunities for a candidate like Bernie Sanders."
Although liberal organizations aren't backing Sanders, he does have a small but devout following in places like Iowa and New Hampshire, critical states in the presidential nomination process. Sanders regularly draws well at small house parties and town halls across both states, bringing out people who see him as their Clinton alternative.
A majority of Democratic voters, though, don't feel this way. In a March CNN/ORC poll, Sanders garnered 3% of the vote compared to Warren's 10%. When you remove Warren from the poll, Sanders jumps by 2 points to 5%, but Clinton jumps by 5 points to 67%.
"I like his views, I like him a lot," said Timothy Horrigan, a liberal New Hampshire representative, before listing all the ways Bernie isn't Elizabeth.
"She is more plausible as a president," Horrigan said. "They have similar messages but they have different styles, and her style appeals to a lot of people. She is sort of like Hillary without all the baggage and just a stronger progressive message."
Many in the progressive movement are uncomfortable with questions about why they are supporting Warren over Sanders.
"We don't have more to say beyond the statement I sent earlier," a spokesman for Move On said after CNN asked a number of questions on the issue.
There is some hope in the liberal movement that the focus on Warren and the issues she represents will help Sanders, too.
Adam Green, co-founder of the Progressive Change Campaign Committee, said Wednesday that Warren "has been a galvanizing force for economic populism" and is someone who is "symbolic of the rising economic populist tide in America."
"Those are just the facts," the liberal organizer added.
By implication, though, Sanders is not that. Though Green said the independent senator is "very much in line with the goal that many progressives have," he acknowledged that Sanders' 2016 campaign would need to ride the "economic populist tide" that Warren symbolizes for many liberals.
"The rising economic populist tide can be ridden by many people," he said. "And I think that any politician who is smart will try to ride that tide."