LONDON -- It was a speech months, even years, in the making. When it was finally delivered, Prime Minister David Cameron had essentially reset the tables for what has the makings of a giant political poker game -- for himself, Britain, Europe and his Conservative Party.
By pledging a referendum within five years on Britain's European Union membership, and warning that voting to leave would be a "one-way ticket," Mr. Cameron embarked Wednesday on a gamble with few precedents in modern British politics, and one few commentators saw him as likely to win.
Aside from being deeply unpopular in Europe, where it met with an immediate rebuff, Mr. Cameron's speech contained many daunting elements as part of his odds-against challenge -- an imposing maze of them at home.
One forum sure to support him after the speech was the Conservative Party. For more than 30 years, through periods in power and out, the party has been rent asunder by divisions over Britain's role in Europe. A strident bloc of traditionalists deeply suspicious of European involvement have long been primed for a showdown with a centrist group, currently arrayed behind Mr. Cameron, that favors a continuing, if reduced, role in Europe. It is a split that blighted the prime ministerships of Margaret Thatcher and John Major, contributing to the political demise of both, and it has shadowed Mr. Cameron since he became party leader in 2005.
Conservatives gave the 46-year-old prime minister a clamorous reception in the House of Commons after his speech. Both pro- and anti-Europeans appeared to see in the referendum an expedient behind which they could unite, since it offered both the prospect of a decisive resolution of the issue, at least for this generation. That reaffirmed the view in the opposition Labour Party -- and among the Liberal Democrats who are Mr. Cameron's partners in the coalition government -- that the referendum plan was, in the words of Ed Miliband, the Labour leader, driven "not by the situation in Europe, but by the situation in the Tory party."
For the Conservatives, there was also the hope that the referendum would blunt the challenge of the United Kingdom Independence Party, whose populist advocacy of an end to Britain's 40-year membership in the European bloc has made increasingly threatening inroads into the Conservatives' political base. The Independence Party, at about 10 percent in many polls, has ridden a tide of disillusionment with the intrusive nature of the bloc's insistent flow of "directives" about everyday aspects of life -- the "sclerotic," "one-size-fits-all" decision-making that Mr. Cameron blamed for reducing popular support for remaining in Europe to a "wafer-thin" margin.
With their public support lagging because of the continuing recession and the deep unpopularity of the government's austerity measures, many Conservatives have come to see a referendum on Europe as a means of neutralizing the Independence Party threat and keeping alive the hope, however elusive by current poll numbers, that Mr. Cameron can win a Conservative majority in the general election in 2015.
Should he lose that, or win with just a plurality, not a majority, Mr. Cameron's referendum plan would most likely collapse. His current coalition partners, the Liberal Democrats, have said Britain could not face the "grinding uncertainty" of a five-year wait for a referendum.
It was a theme taken up after Mr. Cameron's speech by leading British businessmen, who said foreign investors might begin moving their British-based manufacturing operations in Europe if they saw Britain's current tariff-free access to the continent's 500 million consumers under threat.
But Mr. Cameron appeared hopeful that he could emulate Harold Wilson, the Labour prime minister who renegotiated Britain's role in Europe and won a decisive endorsement for the resulting deal in a referendum in 1975, the only previous time Britain has put the issue to a popular vote.
"There will be those who say the vision I have outlined will be impossible to achieve," Mr. Cameron said in his speech. "That there is no way our partners will cooperate. That the British people will set themselves on a path to an inevitable exit. And that if we aren't comfortable being in the E.U. after 40 years, we never will be."
Then he added, "But I refuse to take such a defeatist attitude -- either for Britain, or for Europe."
Correction: January 24, 2013, Thursday
This article has been revised to reflect the following correction: An earlier version of this article misstated the given name of the head of Britain's Labour Party. He is Ed Miliband, not David.
This article originally appeared in The New York Times.