NEWS ANALYSIS : British Tories Facing Nasty Political Season : Europe: Defection and a vibrant new Labor leader threaten Conservatives’ long grip on power.

It is always a proud moment when the Gothic oak doors at Westminster open on a new session of the mother of all parliaments. This year, though, 651 legislators lounging on green benches seemed wed to a raucous Cain vs. Abel drama.

After a long midyear break, Parliament reconvened this week as Big Ben tolled 3 p.m. Nineteen minutes later, through those doors strode Alan Howarth, the first parliamentary member of the Conservative Party to switch to the opposition Labor Party.

Tabloid reporter Paul Callan recorded the historic moment. See if you can guess his politics: “The only noise came from Labor’s baying benches--a terrible combination of shrieking, roaring, guttural mutterings, animal noises, raspberries and the general Goon Show sounds that these days pass as socialist parliamentary style.”

So begins a visceral new political season in a country on the cusp of change. Mirroring renewal under way across much of Western Europe, a party long in power seems on its way to the showers.

Prime Minister John Major would not agree, but as Howarth defected--er, followed his conscience--the Establishment Conservatives are being badly outshone by their Labor opponents. Sixteen years after Margaret Thatcher came to office, pollsters give Labor a lead of nearly 30% over her heirs.

Major need not call new elections until spring, 1997, but with Howarth’s departure the Conservative parliamentary majority is now razor-thin. On actuarial grounds alone, Major might not last: Conservatives lately have demonstrated a unanimous inability to win by-elections called to replace members of Parliament who have died in office.

Annual party conferences on the eve of the parliamentary season drew the battle lines for a restive pre-election period. Major is playing for time, British analysts say, believing that the longer he delays an election the better his chance of checking Labor’s momentum.

In the meantime, Major is nudging the Conservatives to the right. The idea is to distance them from Labor opponents marched to the center by Tony Blair, a dynamic and young party leader who even many Conservatives expect will be the next prime minister.

Populist-tinged calls to arms well known to Americans will ring out as Major tries to win back disaffected “middle England” voters. Looking for a crackdown on crime, and for better education and health services, stronger defense and smaller government? Conservative’s for you.

Promising economic policies that will attract enough investment to make Britain the “enterprise center of Europe,” Major even hints there could be tax cuts when the government’s new budget is formally presented to Parliament next month.

By contrast, Blair, who made what analysts called the best speech of his life at the Labor conference this month, is consolidating the respectable new image of a party once stirred and shaken by its left wing.

“I didn’t come into politics to change the Labor Party. I came into politics to change the country. And I honestly believe that if we had not changed, if we had not returned our party to its values, free from the weight of outdated ideology, we could not change the country,” Blair, 42, told cheering supporters.

The Britain that Blair promises is a revitalized, reach-for-it nation underpinned by traditional family values and wed to a new future by its wits and technology: He promises every school kid the chance to surf the Internet.

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Blair’s selling points to voters parallel Major’s, with a bow to trade unions that still make up a key part of party support. Crime, health, education, defense, jobs are all variations on a theme.

But where the Conservatives would cut taxes, Blair promises windfall taxes on the profits of privatized industry, which he says would raise about $7.5 billion. Major would continue to sell off the nation’s railroads; Blair would restore some sort of public ownership.

Both parties are cautious about moving Britain closer to union with Europe. Major won’t accept fully open borders between European nations; Blair is in no hurry to join any single currency.

But both parties will be jostling for advantage when President Clinton visits here at the end of November to reassert the U.S.-Britain special relationship.