(cnn) she's staring down the worst polling numbers of her campaign, struggling to overcome the scandal surrounding her private emails and grappling with persistent chatter that vice president joe biden might challenge her.









but hillary clinton has one key advantage going into primary season: a firm perception that she's the only democrat who can keep the white house.









"except for hillary who is there?" said brent budowsky, a democratic insider who worked for party luminaries including former sen. lloyd bentsen. "there is really no one else at this moment who has a chance of winning. that is a problem that democrats face -- that is the problem progressives face.









he added: "if for some reason she dropped out, there would be chaos."









as progressives swoon for bernie sanders and biden allies float trial balloons and even rumors of an al gore comeback swirl, budowsky's sentiment points to the reality at the heart of clinton's 2016 efforts. while grassroots democrats might not love the former secretary of state, who often seems to the right of the party's increasingly liberal base, they appear ready to make a pragmatic choice that she is their best -- or only -- hope on election day.









though her lead has narrowed in some instances, clinton is still on top of most polls of democratic primary voters. and she beats potential republican nominees in most match-up surveys.









such is the power and aura of the clinton family that few democrats will go on the record as criticizing the former first lady and many say they respect her and will ultimately support her if she is the nominee.









but from multiple conversations with democratic activists, it's clear that suspicion lingers over her true beliefs and instincts. some fear, for instance, that she would prove to be more to the right of her current political position once in the white house.









all the love in a party that tends to fall hard for its favored candidates is going to sanders. and after all, there is a precedent -- 2008 -- for clinton's primacy as her party's apparently unassailable front runner being undermined by a grass roots revolt.









new polls are adding to the jitters.









a survey this week for the first time put sanders in the lead in a key nominating state -- new hampshire -- and clinton's negative ratings continue to rise over the email controversy that has haunted her candidacy and looks set on to drag on for months, giving a torrent of material for the gop to brand her secretive and shady.









clinton's presidential campaign is hardly setting the party on fire either. she's failed so far to recapture the barnstorming persona which prolonged her battle against barack obama for the party nomination seven years ago.









and clinton often seems a ponderous campaigner -- the undeniable connection she has with individuals does not translate easily to the campaign trail and she frequently suffers by comparison with the political magnetism of her husband -- former president bill clinton.









as she tries to prove that despite her wealth and life in the political bubble she's in touch with everyday americans, clinton's choice of campaign events also makes it hard to inspire.









for instance, on friday in iowa, clinton held a wonkish event on cutting college debt -- part of an emerging attempt to lay a policy foundation under her presidential bid. asked later about her repeated focus on mental health -- one of the core of issues on which she likes to focus in depth -- she was unapologetic.









"i think a president should try to help people have better lives," clinton said, defending a strategy of narrowing in on issues that preoccupy everyday voters rather than big venue campaign events like the ones with which sanders is packing out sports arenas.









the former secretary of state has also sometimes seemed an uneasy fit with progressives in a party that has undeniably moved left since 2008. she's dodged taking a stand on issues that fire up the party base in united opposition like the keystone xl pipeline and obama's push for a pan-pacific trade deal -- a pact she supported as secretary of state.









some democrats are increasingly worried following clinton's handover to the justice department of the private server she used as secretary of state, which is at the center of a controversy over classified intelligence.









there is so far no evidence that clinton is the target of an investigation or could face criminal charges and her campaign says she never sent email with information that was classified at the time. but the latent fear of the unknown may be one reason why clinton communications chief jennifer palmieri sent an email to the democratic frontrunner's supporters this week assuring them the so-called scandal was mere campaign "nonsense" trumped up by republicans.









some senior democrats believe it's too early to push the panic button for clinton.









"it's in the dna of democrats to worry and to freak out and a lot of folks out there are worrying and freaking out," said mo elleithee, who worked on clinton's 2008 campaign and now leads the georgetown institute of politics and public service. "give it a couple of months. if the dynamics don't start to improve, then you start to worry but at this point there is no reason to worry."









there's another reason why the clinton campaign is unlikely be to raising the alarm over nervousness in party ranks.









despite sanders' rise -- he's basking in adulation and drawing huge crowds for a campaign based on blasting wall street, demanding campaign finance reform, free college, a battle against climate change and universal healthcare -- many party insiders doubt america is ready to elect a 75-year-old self declared socialist as president.









and while biden is beloved in the party, stirs deep sympathy following the death of his son beau from a brain tumor, and enjoys the stature of office, there are still questions about his electability.









that's not stopping supporters of the vice president -- who wrapped up his last presidential campaign in 2008 after barely registering in the iowa causes -- from laying the groundwork for a possible run.









a draft biden movement has kicked into high gear, drawing more donations and support, at a time when reports say biden is considering whether there is a path to run in 2016.









the group took in roughly $200,000 in the first quarter and has amassed almost 200,000 signatures.









but, those figures pale next clinton's $45 million haul and the 18 million votes she got in 2008. that current landscape is part of biden's calculus as he mulls challenging clinton.









for the last week he has been vacationing in south carolina and reaching out to supporters to gauge interest.









"if he does decide to enter the race, he will add more gravitas to the democratic field," said jon cooper, the national finance chairman or draft biden 2016. "he has a strong capable hand and is ready to lead on day one without any learning curve."









that decades long resume, however, could also be an achilles heel as polls show that voters are pining for outsiders and fresh faces.









aside from biden, the democratic bench is scarcely populated by alternative power players. two other democrats in the race, former maryland gov. martin o'malley and former virginia sen. jim webb, might pose tricky moments for clinton in democratic debates. but they're the longest of long shot challengers.









meanwhile, the next generation of democratic presidential possibles -- like new york governor andrew cuomo and senators elizabeth warren and kirsten gillibrand -- have yet to emerge, partly because of the prohibitive presence of clinton as the heir apparent.









any late entrants to the race at this stage would run headlong in to the clinton machine.









the campaign has quietly been building the kind of comprehensive grassroots voter identification structure in early states to swell her delegate count that she lacked in 2008.









and she's consolidated her support among democratic party elites -- a barometer of political strength that history suggests is just as important as polling in early states in the summer before nominating votes are cast.









on friday, clinton trumpeted the latest significant endorsement, welcoming the support of former sen. tom harkin, a liberal iowa political icon.









"i have had the privilege of knowing hillary clinton for a long time. she and i share many of the same deeply-held beliefs," harkin said, in a statement that may help insulate clinton from claims she is aloof from the economic woes of many americans.









as often happens with clinton, however, over a long political career marked by fierce fights with republicans and questions about her trustworthiness, good news shares equal billing with unflattering headlines.









it's that trend that has some democrats worried, and looking with some trepidation towards clinton's prospects in an eventual general election.









in july, a quinnipiac university poll of three swing states<u+2014>colorado, iowa, and virginia<u+2014>showed clinton lagging behind possible gop opponents, marco rubio, jeb bush and scott walker. as she has battled bad headlines about her personal e-mail account, her negative ratings have spiked in those states and her trust numbers have also taken a hit.









a cnn/orc poll of democratic voters in iowa released this week shows that clinton still beats sanders overall, yet voters see the vermont senator as more trustworthy. and since july, sanders has attracted cumulative crowds of 100,000 people with his soak-the-rich rhetoric.









"hillary clinton has been doing well in iowa but her people there are rattled by the bernie sanders crowds which by iowa standards are quite large," said david yepsen, who has covered iowa politics for decades. "the polls look good for her now but we are six months away and i've seen things change rapidly. the drip, drip, drip, drip, is causing some concern."
clinton's best defense -- there's no one else
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