Just a day before Britons go to the polls, politicians and parliamentary experts are getting ready to answer a tricky question they haven't faced in decades: Under the country's unwritten constitutional rules, who gets the keys to No. 10 Downing Street in the event of an inconclusive result?
As the three main party leaders crisscrossed the country in the frenetic final hours of campaigning Wednesday, polls suggested Thursday's vote would produce a hung Parliament, in which no party wins enough seats to govern outright.
Britain looks destined for a period of wrangling instead of the usual day-after-an election ritual, which sees the victorious party leader drive to Buckingham Palace, where Queen Elizabeth II asks him or her to form a government.
"The normal thing is someone wins, someone loses, the guy who loses will resign by lunchtime and will advise the queen to call for the person who's won," said Peter Riddell, senior fellow at the Institute for Government, said Wednesday.
Maybe not this time. This four-week campaign has been the most unpredictable in years. Early on, many predicted outright victory for the Conservatives under David Cameron. Then, the third-party Liberal Democrats surged on a charismatic television-debate performance by leader Nick Clegg that tapped into voter dissatisfaction with the two big parties. Struggling Prime Minister Gordon Brown's fortunes sagged further when he was caught on microphone calling a voter a "bigoted woman."
With up to four in 10 voters saying they remain undecided, no one wants to predict the election's outcome.
Whatever happens, Gordon Brown remains prime minister until he resigns, an act which could come as early as Friday.
Convention holds that, in the event of a hung Parliament, the sitting prime minister gets the first chance to try to form a government — even if his party wins fewer seats than the opposition — by making a formal coalition or a looser alliance with another party. But it's likely that the deeply unpopular Brown may not be able to muster enough support in parliament to cling to power.
That opens the way to a potentially chaotic few weeks in which Britain's centuries-old parliamentary system could come under unprecedented pressure and scrutiny amid demands for an overhaul of the electoral system.
"Brown gets first dibs at trying to establish a coalition," said attorney Gavin Millar, an expert in constitutional law. "If he can't do that he has to resign and the queen invites the next one down the constitutional pecking order, which would be Cameron."
Britain's so-called first-past-the-post parliamentary system usually produces a clear result: One party gets more than half the seats in the House of Commons — 326 is the magic number — and its leader becomes prime minister.
If one party gets the most seats but less than half the total — as the polls suggest will happen on Thursday — it's known as a hung Parliament, and things get a little complicated.
The most likely scenario is for the Conservatives to gain the most seats, but less than a majority. Labour will probably have the second-highest number, the Liberal Democrats third, with a handful going to smaller parties.
If Brown feels his position is strong enough, he could try to make a deal with the Liberal Democrats or others to get a working majority, even if Labour has fewer seats than the Tories.
If this proves impossible, the Conservatives will get a chance to form a government.
In either case, the Liberal Democrats are like to play kingmaker — and demand electoral reform as the price of their support. The third party has had a good election, with some polls showing them battling Labour for second place. They are frustrated that the existing system means their share of the vote will likely not translate into more than 100 seats in the 650-seat Commons, and have long called for proportional representation.
How long could the period of uncertainty last? Probably days, but possibly weeks. In theory Brown could hang on until May 25, when the queen in her diamond-studded crown is due to deliver a speech outlining the new government's priorities. That speech is then subject to a vote — and if the government lost, it would have to resign.
The ball would then be in Cameron's court. If he still couldn't form a government, the political parties would ask the queen to dissolve parliament and call a new election.
The last time a British election produced a hung Parliament was in 1974. Conservative Prime Minister Edward Heath got fewer seats than Labour but was given a shot at forming a coalition with the Liberals. When he failed, he resigned and the queen asked Labour leader Harold Wilson to form a government. His minority administration staggered along for about six months, when Wilson called a new election, which he won.
Britain's two big parties — and the financial markets — dislike the instability that comes with a hung Parliament, and the uncertainty makes some observers nervous.
"Because there is no written constitution, there are all kinds of games that will be played — people can make things up as they along even though there are conventions," said Steven Fielding of the Center for British Politics at Nottingham University. "These are uncharted waters."
Although Britain has no written constitution, senior civil servants have been preparing furiously to lay out the rules and avoid market-rattling uncertainty in the event of a hung parliament.
The country's top civil servant, Cabinet Secretary Gus O'Donnell, has visited New Zealand, which has had a hung Parliaments for more than a decade, and civil servants have pretended to be bickering politicians in role-playing scenarios and drawn up guidelines detailing what should happen in such cases.
Vernon Bogdanor, professor of government at Oxford University, said this election won't produce a constitutional crisis, though it could lead to "political and economic difficulty."
"We are facing a large budget deficit, and it's not clear we will get a government with the authority to do it that would satisfy the markets. The markets want a clear, quick decision."
And the queen's role, as she awaits a visit from her latest prime minister-to-be? Basically, to sit and wait. While it is the monarch's job to invite a politician to become prime minister, it's the parties in Parliament who decide who that should be.
Millar said the whole process of constitutional precedent was designed to "prevent the queen being the kingmaker."
"She's a constitutional monarch," he said. "She's not supposed to get involved in politics."