Every so often you read an article-- usually written by an aide to a former president--urging that we change to a parliamentary system of government. Presidents can't govern, the argument goes, without an assured majority in Congress; nothing can get done.

Don't believe a word of it. For although we don't like to say so out loud, we already have a parliamentary government, one in which the functional equivalents of key ministries change hands about as often as they do in Italy. And it does get things done, more quickly than we usually realize.

In the two years and three months of the Reagan administration, we have already had three changes of government, each in response to public opinion and each, for its time in power, commanding a majority in Congress. Consider them in turn:

The Stockman government. Former John Anderson aide and John Connally backer David Stockman, all of 34 years old, arrived at the Old Executive Office Building in January 1981 with something no one else in the Reagan administration had: a detailed knowledge of how the federal budget worked. That knowledge made him the effective head of government at a time when the president and the voters who elected him wanted a significant reduction in the size and scope of the federal government (except for defense). Stockman made policy; others ratified it as surely as the British Parliament ratifies the decisions of Margaret Thatcher. At one point, Stockman was even able to get the nominally Democratic House to pass a bill with changes scribbled in pencil in the margins.

The Stockman government, by its nature, couldn't last: Stockman could not maintain his monopoly on information forever. Nor did conditions stay the same. As the economy failed to revive and the deficit widened in the fall of 1981, opinion around the nation and in Congress turned against Stockman. Then the Atlantic article appeared. In West Germany, a powerful Cabinet minister, Franz Josef Strauss, fell when he tried to suppress a magazine; in this country, Stockman's government fell when he broadcast too much in a magazine. After some confusion, there came to power:

The Baker-Dole-Domenici government. With the president unable to propose a budget that could get a single Republican vote in the Senate, and with no one--not Dan Rostenkowski or Phil Gramm, Tip O'Neill or Jack Kemp-- able to command a reliable majority in the House, power in 1982 went to the able leaders of the Senate. They wrote a budget and raised taxes, all in the face of a hostile White House and a House leadership that took the position toward the Senate Republicans similar to that of Italy's communists toward the Christian Democrats--that they would not support their policy openly but they would make sure it was not obstructed. It was a virtuoso performance on occasion: Bob Dole's assemblage of Democratic votes for specific tax rises and then a Republican majority for his old package was legislative leadership as dazzling as Stephen Douglas' passage through an almost entirely hostile Senate of the Kansas-Nebraska Act. Baker, Dole and Domenici provided Americans with exactly the mid-course correction they asked for when they had a chance in the fall elections. The election results, however, changed the calculus on the Hill, leading, even before the new Congress convened, to:

The grand coalition government. They've had them in Europe: Britain's national government in 1931-35, West Germany's grand coalition in 1967-69. The idea is to yoke the two major parties together to lead the wagon of government where it must go but where no one dares take it alone. This grand coalition, brilliantly led by Howard Baker and Tip O'Neill, already has a record of accomplishment unequalled since David Lloyd George's national coalition in 1916-22 won World War I, negotiated the peace treaties, set up the Irish Free State, made peace with the unions in Britain and ended the undeclared war against Lenin's armies in Russia. In just three months, the Baker- O'Neill grand coalition has:

* Raised the gasoline tax. Remember that the agreement on the gas and roads tax bill was made in the speaker's office, in a conversation between Baker and O'Neill. Reagan only agreed to join them the next day.

* Raised Social Security taxes and cut benefits in the form of raising the retirement age. O'Neill was against raising the retirement age, but now that he let the House have a clean vote on it and lost, he will abide by the result.

* Cut the increase in defense spending to half of what the administration has proposed. This may or may not be a good idea, but there is no way Reagan can get enough votes in Congress to overturn the grand coalition consensus.

* Changed national policy on the environment--a change personified by the departure of Anne Burford and the appointment of William Ruckelshaus--exactly the kind of Republican you didn't find in office in the days of the Stockman government.

In each case, the grand coalition has changed policy in a way likely to produce results the public clearly wants-- though in some cases by means the public has long said it opposes. This is exactly what politicians are supposed to do--and what American advocates of parliamentary government say can't be done. Let's appreciate the grand coalition while it lasts --which probably won't be long. Howard Baker is leaving the Senate, Republicans may lose control there, Tip O'Neill may not be speaker forever, and Ronald Reagan may find a way to seize the initiative and to drive the wagon of government again, rather than just ride on as a passenger.