The presidential primary game is one of momentum, and

Sen. Bernie Sanders (I-VT) is playing it surprisingly well.

Starting from the position of political gadfly and surging to legitimate contender for the 2016 Democratic presidential nomination, Sanders has shocked the stage with record-setting crowds, a massive fundraising haul of over $15 million (which, for a campaign that draws its fundraising from small donors and refuses super PAC contributions, is not to be ignored) and strong polling gains in early primary states such as Iowa and New Hampshire.

While all professionally-run presidential campaign operations -- which one would presume former Secretary of State Hillary Clinton is running -- will keep tabs on the field as a matter of course, top Clinton campaign staff and advisers have expressed unexpected concern about Sanders' candidacy.

“We are worried about him, sure. He will be a serious force for the campaign, and I don’t think that will diminish,” said Jennifer Palmieri, the Clinton campaign’s communications director, in an interview on MSNBC’s Morning Joe on Monday.

Other staff have acknowledged that they were surprised by the momentum Sanders is building in Iowa.

“I think we underestimated that Sanders would quickly attract so many Democrats in Iowa who weren’t likely to support Hillary,” said one Clinton adviser, who spoke on the condition of anonymity to candidly share views about the race. “It’s too early to change strategy because no one knows if Sanders will be able to hold on to these voters in the months ahead. We’re working hard to win them over, but, yeah, it’s a real competition there.”

While Clinton still holds a double-digit lead in both Iowa and New Hampshire, it is clear that a Sanders victory in those early primary states is not out of the question. The fact that Sanders has been able to build the name recognition and network that he has thus far stands as testament to how powerful his candidacy can be if it continues to build momentum -- and I would advise the Sanders campaign to keep the wheel turning, as the road ahead is going to be much more difficult.

While Sanders will undoubtedly hold a strong showing in Iowa and New Hampshire -- and may even win those states -- his campaign will have much more work to do if it will continue beyond the hype enjoyed by most candidates in the time that follows their entry into the race. Some party elites -- and even some of his supporters -- have suggested that Sanders could fade as time goes on.

Even the senator himself has acknowledged that money could become an issue once the contest moves to bigger states, where retail politics is replaced with paid communications strategies like television advertising.

There have also been concerns as to how Sanders will appeal to voters in other states. The senator has done well in New Hampshire, but that is to be expected given its close proximity to Vermont as well as its predominantly white demographic mix. According to national polls, Sanders has thus far been unable to earn more than five percent of the nonwhite vote, which may prove problematic among Democratic primary voters nationwide, of which one-third are nonwhite.

Then, of course, there is the matter of Sanders' general candidacy profile: he is an untested national candidate with very little establishment backing. Clinton's endorsements are a force to be reckoned with, with support coming from all over the nation and within every racial demographic, and is crucial in the early stages of campaigns.

That said, and take it from someone who has worked to secure early support for clients, endorsements do not mean everything. The point of having endorsements is to lend credibility capital to your campaign, and to leverage early support into a strong network of donors and volunteers.

While Clinton may hold a strong support network and may have a vast fundraising advantage, the entire point of having money and volunteers is to use them to contact voters. It seems to this political strategist that Sanders is doing a fair enough job of that with the network he already has, and will only continue to surge as more and more voters become aware of his candidacy.

Will Sanders win the nomination? Only time will tell, but one thing is for certain: he is making an astonishingly good go at it.