Britain is already steeped in a state of deep uncertainty about how exactly it will handle its exit from the European Union—and a ‘Brexit’-related lawsuit being heard Thursday, whose lead claimant is investment manager Gina Miller, threatens to make things even more complex.

At issue in the case is Article 50 of the EU’s treaty, which allows member states to leave the union (click here for the full text). The article’s murky language, which says a state’s exit from the EU must be done “in accordance with its own constitutional requirements,” has left its interpretation up for debate. After all, the U.K. doesn’t have a written Constitution.

The U.K. government, now led by Prime Minister Theresa May, maintains that the nation-wide balloting on June 23, which resulted in a vote to leave the EU, and its own executive powers are enough for it to notify Brussels without parliament’s consent.

Subscribe to The World’s Most Powerful Women, Fortune’s daily must-read for global businesswomen.

But Miller, co-founder of investment boutique SCM Private, is one of at least seven individuals who filed private actions challenging that stance.

Their argument, which goes before the High Court on Thursday, is that the executive can’t actually take the country out of the EU—only an Act of Parliament can do that. The referendum itself was, legally speaking, merely consultative, rather than legally binding.

Miller told BBC Radio 4’s Today show that she’s not attempting to overturn the referendum’s outcome, stating, “We are all leavers now.” Instead, she’s interested in obtaining an answer to “a fundamental legal question about the powers that can be used by the prime minister and whether they can side-step parliament.”

That hasn’t stopped demonstrators from protesting outside the law firm representing her. They’ve appeared near the offices of Mishcon de Reya—the law firm representing Miller et al.—with signs saying, “Invoke Article 50 now” and “Uphold the Brexit vote,” according to The Guardian. Those leading the legal challenge have received much more serious backlash too, including death threats and online abuse. At one preliminary hearing for the case, a judge ordered the identities of some plaintiffs withheld for their own protection.

It’s perhaps a good thing then, that Miller is used to being disliked.

Miller launched SCM with her husband in 2009 after the financial crisis showed her how opaque the asset management industry was. Her firm is built on low-cost passive investment strategies, but also actively manages how the assets are allocated. And she’s gained a reputation as an advocate against hidden fund charges and has decried what she characterizes as flagrant mis-selling of funds. She’s pushed asset managers to disclose exactly how much they charge investors. That activism, she told the Financial Times in April, has sapped her popularity in the profession. At one industry party, three attendees called her a disgrace.

“One of them replied…that my lobbying efforts would bring down the entire City [of London],” she added. “I believe that level of abuse means I am doing something right for investors.”

Whether Miller gets that same kind of redemption in the Brexit case is a matter for the courts to decide. The plaintiffs start making their case Thursday. The government’s response is expected Monday and arguments may carry over to Tuesday. The judgment may not be handed down immediately, but whoever ends up losing is expected to appeal to the Supreme Court.

Because there is such a short time before March, which is the deadline May has set herself for invoking Article 50, the case will skip to the top of the court’s docket. An ultimate judgment against the government could force it to draft legislation to invoke Article 50, which would give members of parliament opposed to Brexit the chance to vote against it. A clear majority of MPs across all parties voted to remain in the EU, according to anecdotal reports, while the official results show that over 60% of electoral districts voted to Leave. A vote on Article 50 would thus force many MPs to go against the wishes of their voters.