On the platform here, speaking to an enthralled campaign audience, Prime Minister Margaret Thatcher cut an imposing figure. Her delivery was commanding, her appearance was impeccable. It was understandable why to the masses of Britain's flag- waving Tory faithful, she has become a symbol of modern political leadership.

"You can only strike your way down," she declared midway through an oration devoted to her mission of reversing Britain's decline, "you have to work your way back up!" And the crowd cheered.

But the next afternoon, at the end of a long day on the road, caught in a swirl of supporters, a few hostile demonstrators, the traveling press and the jumpy owners of a brewery she had just visited, Thatcher looked very different. She seemed smaller and older than her 57 years. Her eyes were red-lined and fatigued, her face mottled beneath heavy makeup. Her campaigner's vigor had given way to mechanical rote.

So much larger than life when seen from that auditorium floor, Thatcher up close turned out to be merely life-sized.

As she nears the end of her probably successful bid for reelection, Margaret Thatcher dominates the national scene in Britain as no other prime minister has done, it is often said, since Winston Churchill. The extent of her sway over other Conservative Party ministers is decidedly presidential in its authority, but with the added plus that a solid majority in Parliament can be counted on to execute her will. Unless the opinion polls are wildly off the mark, that will be even more the case after the voting on June 9.

For now, therefore, Thatcher seems beyond serious challenge as the major force in British politics.

Yet just as her personal presence tends to be enhanced by the spotlights of acclaim--and expert coaching on everything from hair style to the cadence of speech--her performance on the job these past four years is not exactly what it appears to be from a distance either.

The mythology of intrepid boldness--"the resolute approach," the Tories call it--arising from her firmness in guiding Britain to victory over Argentina in last year's Falklands War has to a great extent obscured less stirring aspects of her record. After all, only a few months before the war began in April, Thatcher's approval rating in the polls stood at 25 percent, the lowest of any prime minister since World War II.

In her Cabinet there were rumblings of unhappiness from the "wets," old-line Tories who objected to Thatcher's rigid monetarism, which, they said, was squeezing the country's already sluggish growth and causing undue hardship. The economy was in a quickening downward spiral with industrial production dropping, interest rates soaring and unemployment on its way to record levels.

It seemed Thatcher would eventually have to give in to reflationary pressures, at least on fiscal policy, or possibly be dumped. But then came the Falklands boost putting her well out of reach of most opponents.

The war, Simon Jenkins of The Economist wrote recently, gave Thatcher an issue "which brought out her most distinctive qualities: decisiveness, distrust of critics and doubters, love of simple issues easily conveyed to the public. She had spent three years wrestling with the complexity of economics, failing and yet having to defend that failure in public."

Her accomplishments as a war leader--even of a small, colonial conflict 8,000 miles away--permanently transformed her image. But there was another reason "Thatcherism" came to be more tolerable. For all her oft-stated commitment to radically overhauling Britain's economic structure and social order, her government's efforts toward those goals were actually far more pragmatic than many people in and out of her party had realized earlier.

The program for selling off key nationalized industries, for instance, moved slowly and many heavy money losers continued to be subsidized. Such dramatic proposals as replacing state education with a voucher system were shelved. Restraints were placed on increases in public housing rents. Hers was unquestionably a Conservative Party agenda, but nothing like an across-the-board upheaval.

Nor is this subtlety in political character something new. In a just-published biography, Nicholas Wapshott and George Brock wrote that as education minister in Edward Heath's Conservative government a decade ago, Thatcher's "evangelistic style of promoting policy . . . disguised her caution within the Cabinet and gave her the appearance of being more ideological than was the case."

In fact, as a two-day swing this week through Scotland and northwest England amply demonstrated, Thatcher is seeking to convey--even to Tory audiences--that she has done more to further Britain's time-honored welfare state programs than she has done to curb them. Her speech here, a rebuff to insistent Labor Party charges that she is undermining Britain's nationalized system of free medical care, was clearly intended to be one of the biggest applause lines.

"I have no more intention of dismantling the National Health Service," she asserted, "than I have of dismantling Britain's defenses."

Thatcher's reverence for private enterprise and individual initiative is the core of her oft-stated philosophical belief. But she also proudly invoked the numbers of additional doctors and nurses recruited to the health service during her tenure. She dwelled on the real increases in pensions, benefits to the disabled and aid to war widows.

She even sought to offset the sting of unemployment figures twice as high as when she came to office in 1979 by listing the benefits the jobless are being paid and the attempts to retrain them.

"The reality," said Thatcher, "is that this government has maintained, indeed improved, the social services through the worst recession in 40 years."

According to undisputed official figures, overall public spending has actually increased since Thatcher's election in 1979 from 41 percent of total national output to 44 percent. The figures support the claim that the shares for social security and health care have both been increased more than the rate of inflation, although the demands for service have been rising even faster. Only the added expenditure on defense has been larger.

To finance these programs, Thatcher has had to raise income taxes and insurance contributions by about 7 percent in real terms. The bonanza of North Sea oil revenues has enabled the Conservatives to keep government borrowing relatively low. The drop in inflation, a product in large measure of recessionary doldrums, has given Thatcher an economic triumph to proclaim, although that alone hardly represents an overhaul of the national fabric.

Still, there seems no doubt that this will be remembered as very much the Thatcher era, if more for her style than its substance to date. She has transferred, probably forever, real power in the Tory party away from the grandees of the past, Britain's landed gentry and monied elites, toward the middle classes, like herself, the sons and daughters of shopkeepers.

In her Cabinet, she has increased the influence of self-made men--besides herself, there are no front-rank women--and aimed her appeal at technocrats, entrepreneurs and even the upwardly thrusting working class. Her major targets are the trade unions, whose restrictive practices and power to disrupt industry rank at the top of Thatcher's roll of villainy.

Privately, Margaret Thatcher--referred to as Maggie by friend and foe alike, although not to her face--is a prototype for her party's constituency. Trained as a chemist and lawyer, married to a businessman now in semi-retirement, Thatcher apparently lives in the august confines of the prime ministerial quarters at 10 Downing Street much as she would in a comfortable suburban split-level. "Thatcher lives over the shop, as her father did before her," wrote Wapshott and Brock in their biography. There is no domestic staff connected with the apartment at Downing Street, according to the authors, so the prime minister must cook her husband's breakfast. She considers it part of her duty, they report.

"If I have a second term," they quote her as telling an old friend, "I will have to have some help."