As the number of Republican presidential candidates grows on a seemingly daily basis, the Democratic field is slowly becoming more crowded. It now includes Hillary Clinton, Martin O'Malley, Lincoln Chafee, and Bernie Sanders (I-Vt.).

While Sanders entered the race as a distant second to Clinton, his campaign has gained momentum. Nevertheless, in the months ahead, his campaign will face three particular challenges.

One potential challenge for Sanders could be gaining ballot access across the country.

In recent weeks, there has been a debate as to whether Sanders would be able to appear on Democratic Party ballots, given that Sanders is politically an independent.

New Hampshire, for instance, requires candidates to fill out a form stating their party registration. However, Vermont – like 19 other states -- does not have party registration, so Sanders cannot simply change his party affiliation to “Democrat" before submitting his paperwork.

Yet this technicality is unlikely to keep Sanders off the ballot in New Hampshire. Previous presidential candidates have been able to appear on a party’s ballot in the state because of their espousal of that party’s ideals, or because the candidate has appeared on party ballots in other states. Sanders – who caucuses with the Democrats in the Senate and who has received the party’s nomination in Vermont – clearly satisfies both criteria.

The state’s party chairman has assured the public, "At the end of the day, Sen. Sanders will appear on the New Hampshire Democratic Party ballot.”

A similar debate over ballot access occurred in New York, where officials at first suggested that Sanders would be kept off the ballot by citing the state’s Wilson-Pakula law, which prevents non-party members from receiving a party’s nomination.

Yet after ballot access expert Richard Winger weighed in and observed that New York’s primary does not nominate a presidential candidate – a function performed by the national party at its nominating convention – election officials stated Sanders would be able to appear on the ballot after all.

the latest polls, he is chipping away at Clinton’s leads.

Nevertheless, some contests will be more difficult than others – especially in larger states later on in the campaign, and they speak to a second challenge for the Sanders campaign: fundraising.

Sanders, a consistent critic of the role of money in politics, has sworn not to rely on an affiliated super PAC or to accept donations from wealthy donors. Instead, he has solicited contributions of small amounts – sometimes as little as $5. With an average donation of $40, Sanders has thus far raised approximately $8 million. He aims to raise $50 million before the Iowa caucuses take place on February 1, 2016.

These figures pale in comparison to those of his primary competitor, Hillary Clinton. Since officially launching her campaign, she has already raised $17 million – a figure that does not include the millions raised by her supporting super PACs. Some anticipate that Clinton's campaign could raise as much as $2 billion.

This paucity of funds can inhibit Sanders from establishing a strong presence on the ground in larger states, where organizing a staff and mobilizing supporters will be a greater challenge for Sanders than it will be for Clinton, who leads him in terms of resources and political connections.

Unlike Clinton, who is running a professional and systematic "50-state" strategy, Sanders is instead launching an early-states-first strategy fueled by grassroots support. In recent weeks, he has delivered speeches to large crowds – many of them drawing thousands – in cities such as Des Moines, Minneapolis, and Denver.

Such events will be a significant component of Sanders' campaign, since he lacks the means – and desire – to bombard the airwaves with slick television ads touting his credentials and taking swipes at his Democratic opponents. Sanders has sworn not to run a negative campaign or issue attack ads, so his limited presence in the mainstream media may be to his benefit.

years prior to Hillary Clinton's official campaign announcement, the media treated the lack of declared primary challengers as a sign that her victory in securing the party's nomination was "inevitable."

Given the media's fascination with the Clintons and the politics of celebrity, Sanders and the media will be engaged in a kind of meta-struggle: Sanders will want to talk at length about the issues in his usual forceful way, whereas the media will impatiently interrupt his statements and try to turn the primary contest into a battle of personalities.

Because of his political independence and strictly out-of-necessity association with the Democratic Party, his grassroots and populist campaign strategy, and his hot-and-cold relationship with the media, it is clear that Sanders' campaign is not only about trying to secure the party's nomination: he is also waging a campaign against the "traditional" campaign itself. And that may be his greatest challenge of all.