But as the vote—still pegged as a neck-and-neck contest—is tallied Thursday, it’s becoming abundantly clear that it will be a pivotal moment in Cameron’s political career, and it could—in turn—also decide the future of a lawmaker who’s little known outside the British Isles: U.K. Home Secretary Theresa May.

Cameron proposed the vote as a sort of sure-fire way to “settle this European question in British politics,” but instead the issue has blown up in his face, leading many to call it a disastrous misjudgment. The vote has ended up as a dead heat and the debate over it has deteriorated into dangerous and deadly dialogue, tearing apart his Conservative Party and rendering Cameron the likely loser on Friday no matter which side wins.

The PM has said he will not seek a third term in 2020 and he’s promised to stay put at 10 Downing Street in the immediate aftermath of the referendum no matter the result Friday, but several different scenarios could leave him little choice but to step down.

A vote for Leave would almost certainly force his resignation, if not immediately, then sometime very soon after, given that he has led the Remain campaign. A narrow Remain victory could lead to the same end since Cameron’s scare tactics during the Brexit campaign have angered the Euroskeptics who make up about half of his own political party and could force a no-confidence vote. At this point only a wide Remain victory—which seems unlikely—could spare Cameron from either of these outcomes. Even then, the vote has been too bitter and too close for too long to give him a real chance of reconciling his party’s two warring factions.

There’s been no shortage of speculation about who would succeed Cameron as head of the Conservative party and take over the role of prime minister. The brash former journalist and London Mayor Boris Johnson is seen as the favorite if the U.K. votes Leave, having led the Tory rebels in the Leave camp.

But in the case of a Remain vote, May could be the dark horse candidate to take over the party and consequently become the U.K.’s second-ever female prime minister. May has stood back from the bombastic and often hysterical claims and counter-claims of “Project Fear” (as the Leave camp call their opponents) and “Project Hate” (as the Remainers depict the anti-immigrant rhetoric of the Leavers).

The way she’s positioned herself in the Brexit debate means she comes out relatively well no matter the result. She’s been “sphinx-like,” says Tim Bale, professor of politics at Queen Mary University of London and author of The Conservative Party from Thatcher to Cameron. “No one can say she hasn’t taken a principled position,” he says, and they can’t accuse her of spinning lies to bolster the Remain cause—an accusation that Leavers have leveled against Cameron.

In her one carefully crafted speech during the campaign, May was diplomatic, going out of her way to not alienate either side. She started by saying that her remarks would “not be an attack or even criticism of people who take a different view to me.” And she urged politicians and the public—to no avail—to engage in a sober debate.

So as we approach polling day, and as the country starts to weigh up its decision, let us focus on the future. Instead of debating the peripheral, the ephemeral and the trivial, let both sides of the argument debate what matters. And let us do so in a serious and mature way. Let us concentrate on Britain’s national interest. Britain’s future. Our influence around the world. Our security. And our prosperity. Let us make our decision with the great challenges of the future in mind.

There may be more than one reason for her low profile. Immigration, which the Leave campaign has portrayed as out of control, is one of her responsibilities as Home Secretary, and she has consistently failed to meet the Conservatives’ election pledges to cut net inflows to below 100,000 a year. As such, she may have been as much a liability as an asset to Cameron’s Remain team (she was pressed last week into promising “further reform” on immigration rules).

But in contrast to many of the poses struck during the campaign, her reserve has been natural, even quintessential. The Daily Telegraph, the Tory Party’s unofficial newspaper of record, calls her “the quiet woman of British politics,” while the left-leaning Guardian once compared her to “a calm headmistress in a chamber full of over-excitable public school boys.”

May was first elected to parliament in 1997 by her constituency in Maidenhead, a town west of London. She became the first female chairman of the Tory party in 2002 and in 2010 Cameron appointed her the second woman to be Home Secretary, a post that oversees counter-terrorism, prisons and the police as well as immigration. She’s now held that job—one of the hardest in government—for seven years, becoming the longest serving Home Secretary in more than a century. And while she has failed to meet the immigration targets, she has at least spared the U.K. the kind of attacks by home-grown jihadists that have shaken neighboring France and Belgium. She has also emerged relatively unscathed from an inquiry into child abuse by past members of government.

In recent months, May has been most prominent for proposing what’s been dubbed the “Snoopers’ Charter,” a piece of legislation that gives police and other public agencies more leeway to monitor civilian communication in an effort to combat extremism. The measure prompted vigorous pushback from Silicon Valley and from civil liberties advocates who say it casts too wide a net and facilitates the kinds of government spying exposed at the National Security Agency by Edward Snowden. May agreed to an independent review of the still-pending surveillance bill last month.