My parents celebrated their golden wedding anniversary last Saturday, and they did it in style, with three generations of family from both sides of the Atlantic and a couple of hundred friends. Of the many more who could not come, one wrote a particularly nice letter that included the following sentence: “The point about marriage is not that it can ever be without its ‘ups and downs,’ but that the commitment of two people to each other and their love for each other, sees them through it all.”

The man who wrote these words was Tony Blair. He knows what he is talking about. His own marriage has survived 10 years in Downing Street. The normal trials and tribulations of raising four children have not spared the Blairs.

Such experiences either tear a family apart or leave it stronger. The sincerity of Mr. Blair’s words in his letter was plain. Their marriage is the rock on which he and his wife have built their family. The problem is not what the prime minister practices, but what he preaches — or is not allowed to preach — about marriage. As it happens, marriage has suddenly become the focus of political debate in Britain.

The Conservatives are invigorated by the imminent replacement of Tony Blair, a prime minister who plays all their best tunes better than they do, by one who seems politically tone-deaf — Chancellor of the Exchequer Gordon Brown. The fresh-faced Tory leader David Cameron is enjoying a head start in the polls over the lackluster Mr. Brown, and this has emboldened him to launch a spirited defense of marriage, including an admittedly vague proposal to remove the fiscal barriers to marriage erected by Mr. Brown.

The only surprise about Mr. Cameron’s move — which amounts to not much more than declaring his undying allegiance to motherhood and apple pie — is that anybody was surprised. For ordinary people, who by now are used to Mr. Cameron throwing out the conservative baby with the reactionary bathwater in the name of “modernization,” it was a pleasant surprise. For the liberal oligarchy, which controls the press, the bureaucracy, and the culture, it was a rude awakening.

The facts show that the families under greatest pressure are neither single parents nor cohabiting couples, let alone homosexuals, but married people with children. They pay the most taxes, have the greatest responsibilities, and receive the fewest handouts. No wonder so many decide not to marry and so many marriages fail. To the moral and legal commitments and sacrifices that marriage requires, Mr. Brown has added a gratuitous financial burden that hits the poorest hardest.

Tony Blair knows all this, of course, but he dare not repudiate Mr. Brown’s entire policy, which is based on the idea that the fiscal system must be morally “neutral”: tax breaks, subsidized housing, and other forms of welfare should not favor marriage. The impact of welfare on the morality of the recipients is ignored. So we have the strange spectacle of Mr. Blair, whose appeal is largely due to his wholesome family values, unable to counterattack against Mr. Cameron, whose spoilt-brat image is distinctly less attractive to aspiring families.

Mr. Blair’s frustration at his inability to prevent the Tories monopolizing marriage is shared by many of his colleagues. The most popular British prime minister of modern times has sacrificed his most cherished ideal — marriage — on the altar of party dogma, for the sake of a successor who will squander his legacy. In effect, the Tories are now stealing Mr. Blair’s clothes, just as he once stole theirs.

How has it come to this? In an interview Mr. Blair expressed regret that, shortly before his third election in 2005, he announced his intention not to fight a fourth election, thereby giving up a major advantage of his office — unlimited tenure. As his self-imposed deadline for handing over to his successor — next October’s party conference — looms, power has been draining away from him.

Yet Labour politicians are not all fools. They can see that they are heading for defeat under Mr. Brown, and many would like to find an alternative. So far, no credible challenger has come forward, but necessity is the single parent of invention. One Labour politician told me that he has quietly been trying to persuade the prime minister to stay on. To the objection that Mr. Blair promised his party that the 2006 Labour Conference would be his last, my interlocutor has a brilliant reply: “Abolish the party conference!”

Alas, it won’t happen. Mr. Blair, still only 54, will leave Downing Street in the midst of a war whose outcome is uncertain. Not since William Pitt the Younger died in 1806, nearly a decade before Waterloo, have the British people been left leaderless at such a critical time. Pitt’s famous last words were: “Oh, my country! How I leave my country!” Tony Blair is more likely to leave 10 Downing St. in a limousine than in a hearse, but his departure will one day be mourned nonetheless.