Britain's ruling Labor Party is beset with a nagging crisis, the slow, relentless departure of writers and thinkers who give the party its intellectual tone.

The latest defector is Paul Johnson, former editor of New Statesman, the nation's most influential socialist journal. Unlike others who slipped away sadly and quietly, Johnson left with a scorching 4,500-word blast that accuses Prime Minister James Callaghan of taking the country toweard "Auschwitz and Gulag."

Labor, wrote Johnson in the magazine this week, has become "a collectivist party" dominated by "union bosses, few of whom have ever believed in liberty and democracy."

Under their aegis, Johnson said, Labor aims "to crowd everyone into the giant firm," crushing individuals and the self-employed. This strikes at "scientists and inventors, writers and musicians . . . the essential creators who keep civilization going," he said. Johnson's tone has struck some here as a carricature of realty, but his parting shot has echoed and re-echoed in the daily press and on the air. The Consernative Sunday Telegraph reprinted it almost in full. An editorial in the Evening Sunday Telegraph reprinted it almost in full. An editorial in the Evening News called it a "significant" landmark.

Johnson, 42, is an Oxford-educated Catholic and a self-proclaimed moral absolutist. Whether a man of his uncompromising views could ever spend a lifetime in one political party is another question now being raised here.

The attention given Johnson, however, does not simply reflect press bias.His exit is symptomatic of a growing disenchantment on Labor's right wrong.

The death last winter of Anthony Crosland, the foreign minister, deprived those who regard themselves as non-Marxist Social Democrats of their most powerful intellectual voice. Crosland's book a generation ago, "The Future of Socialism," brought many like Johnson to the Labor Party.

The Labor rightwing's political leader has been Roy Jenkins, a distinguished biographer and holder of key ministries in several Labor cabinets. He has now virtually abandoned British politics, taking a high-paid post as president of the Common Market Commission.

Another of the same stripe is Lord Chalfont, who was made a peer to serve in a Harold Wilson government. He has now quit the party to write stern warnings against communism at home and abroad, largely for his former paper, The Times of London.

Most recently, Brian Walden, a political scientist who has taught at Yale and Princeton as well as Oxford, gave up his back-bench seat for Peter Jay's old job as commentator on a Sunday television show.

Even Jay, if he were not ambassador to Washington and Callaghan's son-in-law, might have been expected to follow the others. He often provided them with ammunition, repeatedly insisting that trade unions, democracy and economic stability were incompatible.

Woodrow Wyatt, a former minister, has not torn up his party card although he turns out dozens of anti-Labor articles. His text book, entitled "What's Left of the Labor Party," implies that there is nothing left that he likes very much.

The erosion is not limited to blittering, articulate figures. It could weaken the already loose attachment of more obscure right-wing members of Parliament, and loss of six or eight of them could bring Callaghan down. That is the biggest political threat he faces.

Almost all the disenchanted complain of what they see as the growing power of union leaders in the councils of party and government. But, as Johnson observed, the Labor Party was founded by the unions as their parliamentary voice.

The underlying cause of the resignations may lie in the way that politics here - as elsewhere in the West - is marked by untidy compromise, with no clear distinction between the way rival parties behave in office. There are few noble crusades, clear programs or clean political causes to rally intellectuals here, and many feel more comfortable as dissenting critics than as affirming party loyalists.

Johnson, whom The Observer Sunday newspaper called "the best polemical writer in the country," says he detected the "first whiff of disaster" in 1969, when Wilson gave into the urgings of Callaghan and the unions, abandoning a plan to inhibit union bargaining power.

Apart from that, Johnson cites two specific complaints. One is that Labor "legalize(d) the closed shop . . . open(ing) the road to the corporate state" of "Mussolini . . . Hitler . . . Franco . . . Communist despots."

In fact, Labor only made legal again what had been briefly outlawed from 1971 to 1974. The last Wilson government restored the traditional position, enabling unions to bargain for agreements requiring union membership as a condition of employment. About half the labor force here remains non-union.

Johnson's second specific quarrel is with Callaghan's Cabinet minister who went "trooping to the Grunwick demo." He implies that they sanctioned violence, making Labor "turn again to the dark past and harbor the thugs."

Grunwick is a small north London film-developing company that fired a number of Asian workers who struck a year ago for union recognition. It has become a cause celebre for the union movement, whose pickets have clashed several times with police protecting non-striking workers.