The campaign for Britain's May 3 national election, which beings in earnest this coming week, will offer voters the clearest ideological choice between the two major parties here in many years.

Margaret Thatcher's Conservative Party, which enters the campaign with a strong lead in public opinion polls, intends to reduce taxes, public spending, government intervention in the economy and the power of labor unions.

Prime Minister James Callaghan's Labor Party, which has been in power 12 of the last 15 years, wants to increase government involvement in setting wage levels, government investment in a variety of industries, and redistribution of income through the tax structure and government social welfare programs.

If the Conservative Party, which Thatcher has moved considerably to the right during her four years as leader, wins a sizable majority in Parliament, the election could be pivotal for British government. The only other postwar elections to mark such significant shifts in direction for the country were the big Labor win in 1945, the start 11 years of Conservative rule in 1951, and the beginning of Labour's present dominance in 1964.

If Labor somehow overcomes the disadvantage of a weary winter of Labor strife and rising inflation to regain a working majority in Parliament, it would give the party's strong left wing the opportunity to push for further expansion of the governmenths role in Britain's mixed economy.

It is precisely this specter of steadily increasing socialism that motivated Thatcher to wrest her party's leadership from the last Conservative prime minister, Edward Heath, and painstakingly lay the groundwork to make this election campaign her crusade for capitalism.

"I felt, and the Conservative who elected me presumaby felt," Thatcher said recently, "that the next leader of the party must clearly stand up against the direction in which the country had been moving under both previous governments.

"We had moved too far toward a society controlled by the government, too far toward what wasn't, and isn't, my idea of a society that can flourish."

She went on in a lengthy, revealing interview in the Sunday Observer to say that "the free enterprise societies are very much better off materially" and "must be kept better off morally."

"Moral responsibility will not keep going if government steps in and makes all the decisions for you, decisions which you ought to make for yourself. And it is because so many people believe this, that may kind of conservatism is striking a chord way, way, way beyond my own party."

Talking to voters since then, she has argued repeatedly that taxes must be cut not only to give each family more money to spend at its own discretion, but also to increase individual and corporate initiative that could rescue the nation's sagging economy and morale.

This makes hers a moral crusade that includes strengthening Britain's military defense and maintaining "law and order" inside the country. She has said this means controlling not only common criminals, but also runaway labor unions that infringe on the rights of others through closed shops, secondary boycotts and picketing, and other forms of "legalized intimidaion."

Thatcher has made "less taxes and more law and order" the Conservatives' campaign slogan.

Callaghan is not a crusader. He is a pragmatist who gained and survived the top jobs of British parliamentary government, which Thatcher has never held, by following the will of his party or its leaders at the time, rather than rebelling or becoming an ideological leader himself.

As prime minister during the past three years, Callaghan did not move his party's government closer to the center. He also tried to force on the unions, his party's power base, strict limits on wage increases that were accepted for two years and helped bring down soaring inflation.

This past winter, however, union members got tough with him because their wages' purchasing power was continuing to shrink. Long, nasty strikes led to bigger pay raises and a rising cost of living, while productivity and Britain's growth rate continued to fall.

So, just to bring its own supporters back into the fold, Labor now must campaign from the left. Government, Labor argues, must be the partner of unions, not their adversary. Government investment in business is necessary to protect threatened jobs and to create new ones. Government spending cannot be cut without threatening free health care, free education through graduation from college, child and housing allowances, sick benefits and old-age pensions.

Labor is blaming much of the high cost of living on the Common Market, which many on the Labor left never have liked anyway. Britain, one of the European Economic Community's poorer nations, in terms of per capita gross national product, now pays more money into it than any of the other eight members. Most of the money goes to agricultural subsidies for the production of food that Britons must then buy from other EEC countries at inflated prices.

Callaghan and his aides have made much political capital lately by railing at other EEC members in Brussels meeting and threatening to stop or curt British payments to the Common Market budget. Labor will try to turn British voters' dissatisfaction with high food prices against the Conservatives, who have always been more committed to the European Community.

Elsewhere in foreign affairs, the Conservatives, who have always been more sympathetic toward the white-minority governments of Rhodesia and South Africa, will try to capitalize on the Labor government's failure to win approval from Rhodesian government and black guerrilla leaders for joint U.S.-British settlement proposals.

Thatcher has told voters that a Conservative government might find this month's Rhodesian elections, from which the guerrillas are excluded, to be the most fair that can be held under the circumstances. She is sending a Conservative Party delegation to observe the elections, raising the possibility that as prime minister she might lift sanctions against Rhodesia and recognize the new government there, which will include a disproportionate number of whites, who will control some key ministries.

Despite areas of sharp disagreement between the parties, there are however, limits to the amount of changes a new Conservative government could make.

The welfare state cannot be dismantled. Industries like Rolls Royce engines and British Leyland cars would collapse if the government pulled out of them. Increases in military salaries and weapons development mean increases in government spending. Britain will remain in the Common Market, fighting to get a better financial deal from the EEC, no matter who is in power. And political leaders and foreign observers here would be surprised if Britain strayed too far from the United States in its policy for southern Africa or much of the rest of the world.

As for economic policy, the most important issue in the campaign, Thatcher's man alternative to Callaghan's attempts at government wage policy is to control the flow on money in the economy while allowing "free collective bargaining" over wages.

Thatcher is certain, however, to take government as much as possible out of wage bargaining and to make new laws to curb union power and stops strikes that interfere with government services.

She also is certain to cut income taxes, although her plan to offset that loss of government income with consumption taxes could increase inflation to the deteriment of lower-in-come families. The Conservatives also want to tax unemployment benefits, and make it more difficult for people who have refused "suitable jobs" to collect unemployment benefits at all.

Conservative leaders also have answered Labor on the Common Market by promising to be just as tough in negotiations with the EEC. But, said Francis Pym, a leading contender for foreign secretary in a Conservative government, "There is a vast difference between a fearful Labor Party, which is so lacking in confidence in Britain that it dare not commit itself wholeheartedly to the EEC, and the Conservative Party which wants to make reforms in the context of a successful EEC in which Britain plays a leading role."

The Conservatives could be expected to continue close ties to the United States, even though they do not feel as comfortable with President Carter as Callaghan had and Washington does not know what to make of the moralistic Margaret Thatcher.

This is not just an unusually important election for Britain. It is also a crucial election of both major parties. A Labor loss will likely lead to a bloody fight between the party's left and centrist wings over a successor to Callaghan, who is 67. A Conservative loss could doom Thatcher's vision of her party's future direction.

Because she is seeking to become Britain's, and Europe's, first woman prime minister, and because she stepped on some important toes gaining and consolidating power in the party, she knows that she will probably have just this one chance to put the Conservatives back into power. CAPTION: Picture, British Conservative leader Margaret Thatcher attends yesterday's funeral of her aide, Airey Neave, who was killed last week by a bomb in London. With Thatcher is her husband, Denis, right. In a Dublin magazine interview, the leader of the Irish nationalist group that has claimed responsibility for Neave's killing said that his organization would not make any attacks on British politicians before Britain's