In a general election where the unexpected surge of the Liberal Democrats has put all the usual calculations about the contest between Labour and the Conservatives in flux, there has been a morbid familiarity to the campaign of one party that cannot hope to be part of the jockeying for power many pundits foresee after the ballots are cast on May 6.

The British National Party, inheritor of the ideological mantle of Oswald Mosley’s Union of Fascists in the 1930s, can realistically hope to win only one London-area constituency among the 650 House of Commons seats — if even that. But opinion polls suggest that the party will attract significantly more of the popular vote than the seven-tenths of 1 percent it won in 2005.

The party’s rise, such as it may be, can be traced to the same issue — the rapid increase in nonwhite immigration, particularly from the Muslim world — that has recently empowered far-right parties across Europe, notably in France. Britain’s counterpart to Jean-Marie Le Pen, the demagogic French politician who reached a runoff for the presidency in 2002, is Nick Griffin, a soberly suited, 51-year-old Cambridge-educated graduate in history and law.

Mr. Griffin is a fringe politician. But in this election, more than in any other in memory, popular anxiety about the rapid rise in immigration in the 13 years of Labour rule is the ghost at the banquet. It is a political reality strong enough, according to opinion polls, to influence votes in dozens of constituencies, but one that the major parties can afford to address only in the most modulated of keys, and then, usually, only when others raise it on the campaign trail.

To understand that, it is enough to recall Enoch Powell. Forty-two years ago, Mr. Powell, a prominent Conservative, made a speech saying Britain “had to be mad” to admit 50,000 immigrants a year, mostly then from British islands in the Caribbean. He likened the consequences to the “tragic and intractable phenomenon which we watch with horror on the other side of the Atlantic,” the 1968 race riots in America. A classicist, he indulged his passion for ancient history. “I am filled with foreboding,” he said. “Like the Roman, I seem to see ‘the River Tiber foaming with much blood.’ ”

Mr. Powell was promptly sacked from the Conservatives’ shadow cabinet; he left the party and wandered in the political shadows until his death in 1998. His “rivers of blood” speech has stood ever since as a warning to mainstream politicians of the fate of those who raise the immigration issue with overwrought language, particularly with a racist tinge. In 2005, many people thought Michael Howard, then the Conservative leader, crossed the line with his tough language on immigration, further dooming his party to its third straight loss to Labour.

Small wonder, then, that the prime ministerial contenders trod warily when a nonwhite woman in the audience raised the issue at the second of three televised election debates on Thursday.

To nobody’s surprise, each of the three emphasized the need to curb migrant inflows. Nick Clegg, the Liberal Democrat, urged an amnesty for the million or so illegal immigrants estimated to have lived in Britain for 10 years or more, to “get them out of the hands of criminal gangs,” balanced by stricter border controls; Prime Minister Gordon Brown, for Labour, said new identity cards for foreign residents and a points system for immigration applicants had begun to cut the numbers; David Cameron, the Conservative, advocated a cap on entrants from outside the European Union, “to get it down radically.”

But their competing policies were less notable than the care the three took to avoid any shade of prejudice. “The first thing to say,” Mr. Cameron said, “is that we have benefited from immigration; and people who come here and live legally, we should be incredibly warm and welcoming and hospitable and build a strong and integrated country. I think it’s really important to say that, first up.”

One party leader not invited to the debates was Mr. Griffin, though he wrenched the debate back down to street level on Friday when he unveiled the B.N.P.’s election manifesto. It called for “absolutely no further immigration from any Muslim countries, as it presents one of the most deadly threats to the survival of our nation.” Mr. Griffin said Britain was “full up,” and it was time to “close the doors.”

What has given the issue new political weight is the scale of immigration during Labour rule. Extrapolations from government figures suggest that looser regulations adopted in Tony Blair’s early years as prime minister have led to a net inward migration of about two million people since 1997, with a peak of 330,000 in 2007. Many new arrivals have come legally from East European nations in the European Union, notably Poland. But by far the most non-Europeans have been Muslims, who historically have been slower to assimilate than other immigrants.