There are few states where Sen. Bernie Sanders is more popular than the early primary state of New Hampshire. The Vermont senator has for more than two decades represented likeminded northeasterners just across the border in Congress, and he’s well-respected by New Hampshire residents across the state. In recent state polls, Sanders has crept within 10 points of the Democratic frontrunner Hillary Clinton.

But none of that will matter if he isn’t on the ballot, and there’s a slight hiccup on that front.

In order to participate as a Democrat in the New Hampshire primary, Sanders has to declare his candidacy in Concord as a registered member of the Democratic Party. It’s a measure enshrined in New Hampshire law, which clearly states that “the name of any person shall not be printed upon the ballot of any party for a primary unless he or she is a registered member of that party.”

Come late this year, Sanders, along with the candidates including Clinton and Gov. Martin O’Malley, will have to sign a declaration of candidacy that says “I am a registered member of the Democratic Party.”

But Sanders is not a registered member of the Democratic Party. Not only does his home state of Vermont technically not register voters or candidates as belonging to political parties, but the Senator has staunchly maintained he is an Independent, criticizing both Democrats and Republicans for being too beholden to corporate interests (though he caucuses with the Democrats in the Senate). And while Sanders has won Democratic primaries in Vermont, he’s repeatedly declined to be nominated as a party standard-bearer.

Political operatives, both Democrats and Republicans, have questioned whether the Independent senator is technically eligible to run.

“Bernie Sanders is an avowed independent,” said Republican Charlie Bass, a former member of the U.S. House of Representatives and New Hampshire state senator who helped write election law in the state and argues there’s a good case against the Vermont Senator. “He’s using the Democratic Party as a matter of convenience.”

New Hampshire Democrats have also privately discussed whether Sanders’ place on the Democratic ballot could be challenged.

Realistically, it’s unlikely that Sanders will be seriously challenged for his place on the New Hampshire Democratic primary ballot in February. Competing campaigns have little to gain if they’re seen challenging his right to be a Democratic candidate. Even by New Hampshire state law, it’s unclear his candidacy is in jeopardy. It would be unprecedented for a popular candidate to be excluded from a key primary contest on what appears to be a technicality.

Meantime, his competitors for the Democratic nomination say they’re focused on winning New Hampshire.

“In New Hampshire we prepared for a competitive race from the outset, of course we expect Senator Sanders to be on the ballot,” said Harrell Kirstein, a spokesman for the Clinton campaign in New Hampshire.

A spokesman for O’Malley added: “Our focus is on talking with voters about Governor O’Malley executive experience and the detailed policy proposals that he has laid out.”

There are a few places where Sanders’ campaign could hit a roadblock.

Later this year, New Hampshire’s secretary of state, Bill Gardner, will set the official date of the New Hampshire primary, as well as the period candidates must declare. When Sanders declares his candidacy, the secretary of state’s office can theoretically reject his declaration.

“There will have to be mechanism by which Bernie Sanders will be able to show he is a member of Democratic Party,” said David Scanlan, the deputy secretary of state in New Hampshire.

If the secretary of state’s office does accept Sanders’ candidacy, anyone with standing—another presidential campaign or political party, or perhaps even an everyday New Hampshire resident—can challenge Sanders’ candidacy with the state’s Ballot Law Commission.

“Any guy or gal could come off street and do it, the question is then would they have the standing to do it,” said Brad Cook, the chair of the bipartisan commission. “Normally challengers have been one of the political parties challenged the candidate,” he said, adding “we’ve never had an appeal involving the presidential primaries.”

Supposing the commission decides Sanders’ claim to the Democratic ballot is illegitimate, Sanders can still appeal it in federal court. In that case Democrats have promised to go to bat to defend Sanders’ right to be on the ticket, said New Hampshire Democratic party chair Raymond Buckley.

“The reality is Senator Sanders’ name will be on the ballot. The Democratic National Committee has acknowledged him as Democratic candidate for president. If there is a challenge, the New Hampshire Democratic Party has already informed Senator Sanders campaign we are willing to go to court over it,” Buckley said.

The Republicans “want to create a schism between the candidates in the Democratic party,” said Buckley. “It’s not going to happen.”

Sanders isn’t the first presidential candidate to run in the New Hampshire primary who technically has no party membership. Al Gore, Howard Dean and George W. Bush all ran without being registered party members, because like Sanders, they are from one of the 19 states that don’t register candidates’ political parties—Tennessee, Vermont and Texas. But Sanders’ case is different: he has never been elected of a major political party.

Sanders’ campaign says there’s more than enough evidence to allow him on the ballot. Sanders has caucused with the Democrats, won the Democratic primary ballot in Vermont for Senate in both 2006 and 2012 (though he refused the nomination), and is the ranking minority member of the Senate Budget Committee.

“When he filed his candidacy with the FEC he signed as a Democrat. The party chairman of the Democratic Party invited him into process,” said Sanders’ campaign adviser Tad Devine. “I see no realistic way he would be stopped from being a Democratic candidate.”

It’s murky legal territory, though, and it too early to know for sure how a challenge would play out if it occurred.

“I don’t know if there is a single individual that can say how this would be decided,” said Scanlan, New Hampshire’s deputy secretary of state.

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