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If Senator Bernie Sanders wins the Democratic primary in New Hampshire on Tuesday, it’ll likely be thanks to voters who are—like him—not actually Democrats.

The Vermont senator’s lead in several public polls is bolstered by his strong support among independent, or undeclared, voters, who are welcome to participate in New Hampshire’s primary and could make up as much as 40 percent of the electorate.

Beyond New Hampshire, Sanders’s advantage among independent voters could be his secret weapon in the many large, delegate-rich states that allow them to cast ballots in the Democratic primary. Four years ago, he crushed Hillary Clinton among non-Democrats, according to exit polls; in New Hampshire, he won nearly three-quarters of them, which made up the bulk of his 20-point margin of victory.

Generally, candidates care less about who is backing them as long as they’re winning. But the source of Sanders’s strength could both help and hinder his candidacy if the primary turns into a delegate-by-delegate battle heading into the Democratic National Convention in July. The states that hold either partially or fully open primaries include some of the largest in the country—California, Texas, Ohio, Illinois, Virginia, and Massachusetts.

To the Sanders campaign, his support among non-Democrats is an argument in favor of his electability: Only he can excite the younger, less politically engaged cohort of people who aren’t registered with either party and might otherwise stay home or vote for a third-party contender in November. But the same dynamic could resurrect old resentments within the Democratic establishment that a candidate who for decades has refused to run on the party’s banner in Vermont is poised to capture its presidential nomination.

“You know, he’s not registered as a Democrat, to the best of my knowledge,” former Vice President Joe Biden said of Sanders last week. For years Sanders has resisted entreaties to run as a Democrat in Vermont, though he caucuses with the Democrats as an independent in the Senate.

“Can he truly build a majority coalition within the party?” asked Dante Scala, a political scientist at the University of New Hampshire. The tension could be magnified should Sanders enter the Milwaukee convention with a plurality, but not a majority, of delegates that hail disproportionately from states that allow non-Democrats to vote. “I can imagine the response being perhaps, ‘Well, they weren’t real Democrats,’” Scala said.

First, however, Sanders must focus on New Hampshire, where he faces more competition for undeclared voters than he did the last time around, and his advantage is narrower. Fresh off his near-victory in Iowa, Pete Buttigieg, the former mayor of South Bend, Indiana, is gaining in the polls, and he has made a concerted effort to woo independents and what he calls “future former Republicans.” Buttigieg polled even with Sanders among both Democrats and undeclared voters in a survey released Friday morning and overtook him in another poll over the weekend. Former Vice President Joe Biden is also targeting middle-of-the-road voters and recently touted endorsements from 100 New Hampshire independent.

Yet not all independents are the same: Only about half of them in New Hampshire are truly swing voters, while the rest lean strongly to one side or the other, says Liam Kerr, a co-founder of The Welcome Party, a center-left group operating in New Hampshire and South Carolina. The organization’s goal is to entice undeclared voters who typically vote only in general elections to participate in the Democratic primary.

“There are different types of independent voters, and people are speaking in different ways to them,” Kerr told me. Buttigieg and Biden are going after the traditional centrists who vote for both Democrats and Republicans depending on the election and who’s on the ballot. New Hampshire is likely to have more of these voters up for grabs this year. Undeclared voters can choose to vote in either party primary, and while they split between the ultra-competitive Democratic and Republican contests in 2016, only the Democratic primary is a real toss-up on Tuesday.

Sanders’s independent base, on the other hand, is among younger, less engaged people “who maybe are against party structures in general,” Kerr said. He needs them to show up in large numbers, and based on the modest turnout compared to expectations in the Iowa caucuses, Sanders may have cause to worry in New Hampshire. “There’s one thing we know about Sanders: He’s getting fewer votes this time than last time,” Kerr told me. “His share of independents has fallen since 2016.”

Sanders is also relying on the independent vote in the open South Carolina primary on February 29, and he may have some unwanted help. A group of GOP leaders and Tea Party activists have mounted a push for Republicans to vote for Sanders in the Democratic primary, in the hopes of boosting a candidate they view as easier for President Donald Trump to defeat in the fall. While only Democrats and undeclared voters can participate in the New Hampshire primary, the South Carolina contest is open to anyone. (The state Republican party voted to cancel its own primary in deference to Trump.) Republicans could try similar gambits elsewhere. The conservative commentator Hugh Hewitt, for example, said on Meet the Press last month that he would vote for Sanders in the Virginia primary.

Whether those efforts will make any difference is unclear. But Sanders has already demonstrated his strength with voters who have snubbed both parties, and that may prove decisive not only in New Hampshire on Tuesday, but in many states to come.

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