After a bitter, bruising and unusually fragmented six-week election campaign, British voters began to vote Thursday knowing just one thing with near certainty: Nobody is going to win.

Or, at least, polls suggest that no one will win in the way elections are usually decided in British politics, with one party claiming a majority in Parliament and a clear mandate to run the country.

Instead, Thursday’s results could set off a mad scramble for power come Friday morning, as parties joust for position with the public, cobble together alliances with sworn enemies and seek to exploit ambiguities in a set of procedures for determining who governs Britain that could be put to the test as never before.

At the end of it all, either Conservative incumbent David Cameron or Labor challenger Ed Miliband will emerge as prime minister, with Britain’s economic fortunes, place in the European Union and future as a global power all riding on that outcome.

(L-R) Deputy Prime Minister and Leader of the Liberal Democrats Nick Clegg, opposition Labour Party leader Ed Miliband and British Prime Minister and Leader of the Conservatives David Cameron on the campaign trail on May 6, 2015. (-/AFP/Getty Images)

But if the polls are accurate, neither man faces an easy path to 10 Downing Street, and either could end up in charge of a fractious and unstable government that is undermined by claims of illegitimacy. If the voters’ verdict is particularly murky, there could even be another election before the year is out, leading to months of domestic political turmoil at a time when Washington is growing exasperated with its top ally for its increasingly inward focus.

On the last day of campaigning Wednesday, with candidates sprinting across the country in a final appeal for votes, the post-election jockeying was already in full swing.

Cameron took to BBC radio to warn that Miliband was planning a “con trick” to claim power even if Labor doesn’t win the most seats of any party in Parliament. Miliband, meanwhile, continued to insist that his party would govern on its own terms, despite the fact that it will almost certainly need the backing of the Scottish National Party to oust Cameron.

From the start of the campaign to the closing day, the national polls have barely moved, showing the Conservative and Labor parties with roughly a third share of the vote and the rest distributed among the smaller parties.

Those polls translate into projections that indicate the Conservative Party will win more seats than Labor in the 650-seat Parliament, with estimates of 280 or a few more for Conservatives and 265 to 270 for Labor.

But Labor has more potential allies among the smaller parties, which are expected to sweep up a greater share of seats than ever before, and may therefore be better positioned to command a majority — the true test of whether a party can govern.

The second-place party hasn’t gone on to lead the country for nearly a century, but it could happen this time.

Britain's 2015 election is one of the tightest in decades and could lead to potential government gridlock. The Post's Adam Taylor explains the possible outcomes, depending on which parties win the most seats in parliament. (Alice Li/The Washington Post)

However the vote turns out, getting from Election Day to a Buckingham Palace invitation to form a new government could be chaotic. For a society that loves rules as much as Britain does, there are surprisingly few of them to govern the process by which parties decide who should run the country once voters have had their say. Britain lacks a written constitution, and the procedures have evolved over time.

“We don’t really have any rules. We have conventions, and conventions can be broken,” said Simon Hix, a political scientist at the London School of Economics.

Britain got a taste of how messy the process can be after the last election, in 2010, when no party won a majority. Days of secret negotiations and uncertainty followed before then-Prime Minister Gordon Brown resigned and Cameron took over.

In 2010, the kingmaker was clear: The centrist Liberal Democrats were the third-largest party, and they chose to form a coalition government with the largest party, the center-right Tories. This time, it may take three parties or more to get to a majority, and the alliances could be deeply fraught.

Cameron’s most logical coalition partner is his current one, the Liberal Democrats. But the parties disagree sharply on whether to hold a referendum on European Union membership, which could make it impossible to agree on a formal coalition.

Even if they could do a deal, they still may be short of the votes needed to form a government, forcing Cameron to turn to Northern Ireland’s right-leaning Democratic Unionist Party for additional support. The anti-immigration U.K. Independence Party has also said it is open to a coalition with the Tories. But it is unlikely to have more than a handful of seats, and the price for its backing would be high.

If there is a surprise brewing on the eve of the election, it would be a bigger-than-anticipated number of seats for the Conservatives, giving Cameron the opportunity to form a government relatively quickly.

Labor has a standing offer of support from the party expected to be the third-largest in Parliament: the Scottish Nationalist Party. SNP leader Nicola Sturgeon has called on the Labor leader to join with her, the Welsh nationalists and the Green Party in a progressive coalition.

“If there’s an anti-Tory majority on Friday morning, we should work together to keep the Tories out,” Sturgeon told supporters Wednesday, predicting “a watershed election” for her party.

But Miliband has repeatedly ruled out any coalition or even any “deal” with the SNP, on the grounds that the nationalists want to break up the United Kingdom. He has left open an arrangement in which the SNP could support Labor on crucial votes without any explicit understanding.

Analysts say that, despite accusations from Cameron that Miliband would be beholden to the SNP, the Scottish nationalists may have left themselves with little leverage by definitively ruling out support for the Conservatives.

“Labor has more cards than it appears with the SNP,” said Peter Riddell, director of the London-based Institute for Government. “The SNP will acquiesce in a Labor government. Ultimately, they don’t want to be seen as bringing it down and bringing in the Tories.”

Still, there would be serious questions about how much a Labor government could accomplish if it’s relying on SNP votes, and the arrangement may not last long. In a bitter irony for Labor, it’s the party’s jaw-dropping collapse in its traditional Scottish stronghold that has deprived Labor of what might otherwise be a decisive victory.

“It’s almost revolutionary,” said Thomas Lundberg, a political scientist at the University of Glasgow. “It’s one of those seismic shifts that happens maybe once in a generation.”

Whatever the outcome on Thursday, this election appears destined to break a series of records in terms of vote shares that underscore the fracturing of a system long dominated by two main parties.

The figures point to the current state of politics in Britain, one in which disaffection with politicians has translated into more support for minor parties, especially UKIP and the Green Party. Due to Britain’s first-past-the-post voting system, both parties will end up with far fewer seats than their percentage of the vote would warrant. Meanwhile, by capturing almost every seat in Scotland, the SNP will be overrepresented.

Such an outcome could point to deeper troubles for the United Kingdom, ones that transcend the likely turmoil of the next several weeks. With nationalism rising in England and Scotland, and resentment building on both sides of the border, the election could ultimately become one more milepost along the way to a breakup.

“When you’ve got 50 SNP members of Parliament throwing their weight around in Parliament, that’s going to go down badly in middle England,” Lundberg said. “And that can’t be good for the future of the union. The more you look at it, the more you think that maybe this just isn’t very viable.”

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