Urging voters to help prevent "the triumph of Marxist socialism" in Britain, Conservative challenger Margaret Thatcher finally came out swinging this week in her campaign for prime minister in the May 3 national election.

Britain's "slither and slide to the socialist state" under a Labor Party government during the past five years created an "ever-growing dominance of the state with all its despotism and frustration of human happiness," she told a loudly enthusiastic crowd packed into the Cardiff city hall in Wales last night. "The similarities between the Labor and Communist manifestoes," she said, "underline what is happening."

Thatcher accused the Labor Party and Incumbent Prime Minister James Callaghan of making a "shameless appeal to voters to accept our national decline as inevitable and simply make the best of it."

"I am a reformer," she declared, "and I am offering change."

Before this week, Thatcher had sat quietly on her party's comfortable lead in public opinion polls, patiently explaining her promises to cut taxes and government spending and reduce government involvement in the economy and other aspects of society.

Now, with the Conservatives' margin in the polls a bit smaller and her personal popularity once again running behind both Callaghan and her own party, Thatcher has returned to the evangelical ferver that won her the leadership of the Conservative Party and moved it considerably to the right.

On a BBC radio call-in program today, she reiterated her controversial statement of last year that some parts of Britian felt "swamped" by black and Asian immigrants from the Commonwealth.

Explaining that streets on which Britons had lived all their lives had completely changed in character, she told a caller today, "I would expect people in India and Pakistan to feel swamped if enormous numbers of people from here lived in small areas over there."

When the caller said the Conservatives' campaign promise to greatly restrict immigration was making blacks and Asians here feel "undesirable," Thatcher replied that her policy was necessary for the well-being of immigrants already here.

They are entitled to the same rights, responsibilities and opportunities as other British citizens, she said, "but we cannot go on taking in more and more people against the interests of the people who are already here."

Thatcher's statement last year about restricting immigration was followed by a sharp increase in the Conservative Party's popularity in opinion polls, but its also led to accusations that she was insensitive and uncompromising in her beliefs.

Callaghan had been trying to needle Thatcher into being combative in this campaign, hoping she would frighten voters with her strong statements.

In last night's speech, billed as the keynote of her campaign, Thatcher tackled the question of her zeal head on and refused to apologize for it.

"In politics," she said, "I've learned something you in Wales are born knowing: If you've got a message, preach it.

"I am a 'conviction' politician. The prophets didn't say, 'Brothers, I want concensus,' They said, 'This is my faith and vision. This is what I passionately believe. If you believe it, too, then come with me.'"

She called on those who have voted for labor in the past to join her now because "Labor has betrayed those for whom it promised to care." Giving back freedom to the individual, she said, the Conservatives would restore some of Britain's former pride "before time runs out."

Callaghan answered at a press conference today that "the Conservative Party was vacating the centerground of politics and moving to the right." He said voters were not being offered traditional Conservative policies but something different and divisive.

"I'm going for the center ground," Callaghan said, "as well as the left and radical ground."

He tried to dismiss this past winter of labor disputes, which Thatcher has repeatedly emphasized and which had helped give the Conservatives their lead in opinion polls.

"Last winter's disputes were not typical," Callaghan said, "They were an aberration." He said labor unions were in accord on their wage settlements "in four out of the last five years."

Besides, he added, he now had a promise in an agreement the Trade Unions Congress made with his government "that what happened last winter won't happen again" if Labor remains in power. CAPTION: Picture, Thatcher, calling herself a "reformer," campaigns with evangelical fervor. UPI