Former Secretary of State Hillary Clinton is the clear, and perhaps insurmountable, frontrunner for the Democratic nomination with a huge delegate lead over Senator Bernie Sanders.

But the Sanders campaign believes that those numbers may not matter if they can win more states and gain momentum in the remaining primary contests before the convention in July.

Essentially, Sanders campaign is arguing that the "pledged" delegates each campaign has secured in primaries so far aren't really pledged. And that if Clinton has a difficult spring and the momentum is on Sanders' side in the second half of this race, they can get her pledged delegates to support the Vermont senator instead.

"Pledged delegates ... usually and traditionally have voted for candidates for whom they have been elected, [but] under our rules are not bound to do so," Sanders senior adviser Tad Devine told reporters in a conference call this week.

It's a theory Devine has repeated in recent weeks, as the Clinton campaign has continued to expand its delegate lead over Sanders. The former secretary of state currently leads the Vermont senator by about 300 delegates, a lead that will be difficult for Sanders to overcome barring huge victories over Clinton in some of the big states that have yet to vote.

Devine's argument is based on rule 12.J of the Democratic National Committee's convention rules, which says: "Delegates elected to the national convention pledged to a presidential candidate shall in all good conscience reflect the sentiments of those who elected them."

To Devine's ear, that rule doesn't bind delegates, but gives them the freedom to do what they want as long as their votes express the "sentiments" of voters in the primaries and caucuses. If the momentum seems to be going in Sanders direction in the final states, Devine argues, casting a vote at the convention for him would do just that.

According to the Democratic National Committee, Devine's theory conflicts with "the spirit of the law, [but] not the letter of the law." DNC rules strongly suggest that the pledged delegates should vote for the candidate that their state elected them to support, but they do not specifically require delegates to do so.

Devine's argument boils down to this: If Clinton begins to lose major primary contests in the coming months, for example failing to win any states on a big primary night as Sanders did on Tuesday, her supporters and delegates will begin raising questions about her viability. And as the clock ticks closer to a Donald Trump nominating convention, Democrats will grow even more nervous. At that point, Devine argues, some could be swayed to back the Sanders campaign over Clinton despite their "pledged" status.

Still, the chances of this strategy coming to fruition are slim. Democratic candidates vet their delegates before they are confirmed at their various state conventions and have the right of refusal. Many states have not yet locked in their delegates, including for example Iowa which voted in February but won't confirm them until their state convention in June.

The Clinton and Sanders campaigns will be interviewing potential delegates up through these state conventions, only accepting strong supporters that will be difficult for the other campaign to sway to their side. The pledged delegates who make it to the convention in Philadelphia, in other words, won't be undecided voters who could be easily moved by a late surge from campaign. They will be die-hard fans wearing "Bernie is Bae" t-shirts who exclusively drink Bernie Weisse beer and Clintonistas wearing Hillary 2016 undies to complement the Clinton tattoos on their legs.

Barring some major event or scandal, these are not fair-weather delegates.

Devine emphasized that the campaign is not currently making calls to try to sway Clinton delegates, adding that they don't currently have any plan to do so. "We're not out trying to convince anybody to do anything at the moment," he said.

If the momentum swings in Sanders' direction that could change come convention time.

Superdelegates, members of the party elite, are a different story and could ironically be easier to sway if the tide of the election is truly turning before the July convention.

But that too, is a big challenge for Sanders' campaign. The Vermont senator's team believes strongly that the second half of this campaign will benefit them, as the voting states shift away from the South, where Clinton dominated, to the West and Northeast. Sanders is the favorite in some of these upcoming contests, including those in Idaho and Washington state this month. But in some of the biggest, delegate-rich states that vote later in the spring, Clinton has huge leads, including in states like California, Pennsylvania, New Jersey and New York. Sanders' team believes they can make some of these races competitive, but even near-wins or slim victories won't likely be enough to change the momentum significantly in his favor.

Winning the nomination on pledged delegates before the convention would require Sanders to win a lot of these states and not just win, but blow Clinton out of the water. That's a tough task. Building momentum for the campaign without fully overcoming her current delegate lead is a lower bar, but still not an easy one to overcome. It means a number of big wins and a sprinkling of upsets that worry Clinton supporters, something they've done before, but not on this level.

Could things change? If 2016 has taught us anything, it's that anything is possible. But the Sanders campaign is relying on a lot of ifs right now.