Bernie Sanders’ presidential campaign has a challenge ahead of it before Primary Day.

The Vermont senator gets some of his strongest support in New Hampshire from independents. But those same voters could, at the last minute, decide to cast a ballot in the Republican primary.

Undeclared voters are by far the biggest bloc in the Granite State. And because state election law lets them decide to vote with whatever party they feel like on primary day, they are really hard to corral.

But many of them like Bernie Sanders. A lot.

One recent poll that looked at independents found they favor him over Hillary Clinton by a 34 point margin. In another poll it was 55 points.

These are numbers Karthik Ganapathy, the campaign’s New Hampshire communications director, likes to hear.

"Bernie’s message speaks to an incredible sense of frustration out there, that systems in this country aren’t working for average everyday Americans," he said.

But support in the polls is one thing. Getting that support on Election Day is something else entirely.

Dave Paleologos who runs the Suffolk University Political Research Center, says this doesn’t make the campaign’s life easy… but it could be an opportunity.

"Relying on independents is dangerous," he said. "That’s why the onus is on the Sanders campaign to lock up those people, to get them as charged up as possible to get out to vote and to get their friends out to vote."

The trick is that many undeclared New Hampshire voters, no matter what their preferences in the general election, vote in the more exciting primary.

One recent survey estimated that more independents are likely vote in the Republican primary this year, by about a 55-45 spread.

Part of that is because some would-be supporters, like Mark Traeger of Sandown, decide they’re going to cast a strategic vote. They would rather play mischief in the GOP race than cast an earnest vote for someone like Sanders, who they actually support.

Traeger can’t decide between Cruz and Trump; he’s not sure who he thinks has the best chance of losing the general election.

Then there are folks like Bill Smith, of Penacook, who also leans Sanders, but won’t be voting for him.

"I’m a little scared of what’s going on," Smith said. "I’ve been looking for some sanity, I think I might have found it. So that’s where I’m going. I think I might be voting for John Kasich this time in the Republican primary."

According to a Suffolk University poll, some 7 percent of voters whose first choice is Sanders would consider voting Kasich. Another 8 percent would consider jumping the fence to cast a ballot for Trump.

So how is the Sanders campaign encouraging independents to pull a Democratic ballot instead of a Republican one, when there is so much going on in the GOP Primary?

Well there are ads. Sanders’ latest one, set to the tune of Simon & Garfunkel's "America," intersperses images of classic Americana with images of the massive crowds at his rallies. And it seems aimed specifically at independents.

via GIPHY

Josh Lieberman is with Audience Partners, a firm that targets digital advertising to very specific audiences. He thinks this is the kind of ad that can appeal to anybody.

"It’s by far the best ad I’ve seen in months, and it kind of tugs on that of your brain that you know isn’t thinking logically, isn’t thinking angrily, but thinks, 'You know, I live in a pretty damn good country,'" Lierbeman said.

And you can bet Sanders campaign is doing what it can to put this hopeful, non-partisan message in front of targeted digital audiences, by using firms that do the kind of thing Lieberman does.

"I know I can find the audience you tell me to find," he said. "The question is, who is that audience and what is the independent who is going to vote for your candidate going to look like?"

Campaigns are super wary of giving away their strategy, but in the third quarter of last year, the Sanders campaign had spent lavishly – more than $2 million dollars – on digital ads, much of it aimed at independents. By contrast, it spent virtually nothing on TV advertising until November.

Says Ganapathy, the Sanders New Hampshire communications director: We’re not disregarding any potential avenue to reach voters. We’re doing it online, we’re doing through videos, we’re doing it through twitter, we’re doing it on Facebook, and on the phones, we’re doing it at their doorstop. Every possible way we can reach folks we’re doing that."

But whether the Sanders campaign is able to capitalize on its strength with independents may not be up to them. The real question is: How many independents will look to the Republican race and think, "Well, that’s where the real action is"?