WE DON'T NEED a parliamentary system, as assorted columnists, former presidential aides and present Anglophiles so often claim. We don't need one because we already have one. It's just that it's modeled on a different country than critics of our system would like.

The parliamentary system most critics have in mind is the British one, where the party in power selects the prime minister and has an automatic legislative majority for whatever it proposes; its budget is adopted within days and temporarily unpopular measures can always be passed.

Quite a contrast, it is said, with our separate executive and legislative branches, with year-long wrangles over the budget, with presidents often opposed by obdurate majorities in Congress.

But in practice, we already have a parliamentary government. Not the British one -- the Italian one.

Italian politics is often held up to ridicule as comically unstable, with constantly changing ministries, divided responsibility, and splinter parties. But how much different, in practice, is ours? Ministries granting responsibility for various issues are rotated in Italy among feuding politicians so as to assemble a majority in the parliament.

Functional responsibility -- not necessarily the title, but the real decision-making power -- gets passed around here as well, to those strong enough to grab it -- witness, in recent days, the jostling over the Gramm- Rudman and tax-reform legislation.

In a parliamentary system, executive departments are headed by legislators, while in America those are supposed to be separate jobs. But in the past few years executive officer David Stockman, director of Office of Management and Budget, had such control over the legislative process for a moment that he got a House nominally controlled by the other party to pass a bill with his pencilled notations written in the margins; and Rep. Les Aspin (D-Wis.), nominally a congressman, effectively made an executive decision to build a certain number of MX missiles with a certain basing mode.

It is a tradition in Italy to bemoan the instability of the government and the weakness of the parliamentary system, a tradition even more firmly rooted than the tradition here of calling for a parliamentary system.

Yet Italy enjoys buoyant economic growth and cultural vitality, and even has a government that has lasted three years in office. Our own de-facto parliamentary system has produced decisions on important issues and has adopted policies -- Social Security cuts and a gas tax, for instance -- which only a short time ago were considered politically impossible. You may or may not consider these policies desirable, but you have to admit our parliamentary system gets things done.

How this works is not apparent when you regard our system with the usual labels attached. But it becomes clear when you consider who, in effect, has assumed the function of prime minister, who makes policy on the central issue of the day.

Sometimes of course there are interregnums, times when no one makes policy effectively. That was true during much of the Carter presidency.

But since 1980 we have had four functioning parliamentary governments over five years -- fewer than the Italian postwar average, but more than the Italians have had during the same period. Three of them were largely successful. The mettle of the fourth is being tested now: Can it command majorities on the major issues before Congress, and produce results that can be sustained over a long haul?

The first prime minister was David Stockman. As the incoming director of OMB in 1981, he had an effective monopoly of information about how the federal budget at that time worked, a monopoly that enabled him to determine the size and shape of the big cuts in taxes and the budget that Congress obediently passed. The Stockman ministry fell after an article in The Atlantic in which he gave credence to arguments against the policies he had just installed.

Stockman was followed by a ministry effectively headed by Sen. Howard Baker (R- Tenn.) with Sen. Robert Dole (R-Kan.) as ministers of finance and Sen. Pete Domenici (R-N.M.) as minister of the budget. Together they wrote and passed the budget and tax bills of 1982. These established a status quo on budget and macroeconomic issues which has been largely maintained, through wildly varying economic times, ever since. The Baker ministry fell after the 1982 elections, when Republican losses in the House persuaded Baker that the ministry no longer commanded a congressional majority.

The third ministry was a grand coalition government, a combination between leaders of two ordinarily hostile parties, of the sort Germany had between 1967 and 1969 and Britain got in 1916 and 1931. The coalition was formed in December 1982 when Howard Baker walked across the Capitol to Tip O'Neill's office and agreed with him to support a roads-and-gas-tax bill.

Grand coalitions are used in European parliamentary systems to pass measures that neither party alone could support politically; and so it is here. The grand coalition passed within days the gas tax long supposed to be a political impossibility. A few months later, following the promptings of a bipartisan commission that was in close touch with grand coalition leaders O'Neill and Baker, Congress adopted previously unthinkable Social Security benefit cuts and tax rises.

Then the grand coalition hunkered down and awaited the prosperity of 1984, which returned it to office. President Reagan, Tip O'Neill's House Democrats, and the Senate Republicans were returned in the number- one incumbent year in nearly 200 years of American elections.

The big political news of 1985, although it has not been reported as such, has been the breakup of the grand coalition. Ronald Reagan broke up the grand coalition government last May. The question now is whether Reagan can take its place.

Nothing forced him to break up the coalition. It was chugging along just fine, working to produce a budget with a deficit a good deal smaller than what he proposed. Just about every practicing politician believes the deficit is too high, that it is producing a trade deficit and an overvalued dollar which are costing Americans jobs and producing high interest rates. So O'Neill and Dole set to work on a set of budget resolutions involving cuts in domestic as well as defense spending and a freeze in Social Security. The president signed on.

Then crunch! Reagan reversed position. He wanted to keep his campaign promise (extorted from him by Democrats) not to cut Social Security; he didn't want any cuts in Defense Secretary Caspar Weinberger's Pentagon budget; he wanted to keep blaming the deficit on Congress's domestic spending. Most of all, he wanted no tax increase (for a moment he refused to back even a continuation of the cigarette tax).

Suddenly he was acting like a prime minister taming a recalcitrant chamber of deputies rather than a president who was above the fray.

Italians would regard it as ridiculous and dangerous if that happened there: The president in Italy and other parliamentary nations is a ceremonial figure who functions politically as no more than a referee. Much of the time Ronald Reagan seems to have functioned in just this way. But on the big issue he evidently sensed that the grand coalition's underlying policy was to raise taxes; after all, arithmetic tells you there's no other way to whittle the deficit way down. So he decided to derail the grand coalition and take on the duties of prime minister himself.

The rest of the political year has consisted of efforts by other politicians to assemble different governing coalitions, and the foiling of those efforts by Ronald Reagan. The question is whether Reagan can build a stable majority for his own policies.

In the process the focus of politics has shifted wildly from the budget resolution to tax reform to trade restrictions to the Gramm-Rudman-Hollings bill purporting to require an automatic balancing of the budget, and to tax reform again.

The attentive reader will sense that all of these are the same thing in different guise. If you can't cut the budget deficit, then try to cut the trade deficit, or put through a tax bill that will end up raising revenue, or enact an automatic budget-balancing mechanism.

Reagan has had mixed success on these measures. He neutralized the Democrats' initiative on trade. But in embracing the Gramm-Rudman automatic budget-cutting bill, and in embracing so lukewarmly the tax- reform bill crafted by Rep. Dan Rostenkowski (D-Ill.), Reagan may have set a course he can't keep.

He may retrieve success next week or next month. But in the meantime he seems to have staked out positions unlikely to form the basis of a parliamentary majority over the next three years.

Begin with trade and protectionism, which for a moment this fall became the nation's number-one issue. In effect, the Democrats seized control of a ministry and tried to make it the center of attention. Heavyweights Sen. Lloyd Bentsen (D-Tex.), Rep. Richard Gephardt (D-Mo.), and Rostenkowski had come up with a punitive trade bill last summer. House campaign chairman Tony Coelho (D-Calif.) ballyhooed trade as the key to the Democrats' win in the Texas special election in July.

Then Reagan took over the issue. Treasury Secretary James Baker got the Plaza Hotel agreement to lower the value of the dollar. American exports became cheaper, imports more expensive. The problem is not solved but it is eased. And Baker securely holds the trade ministry now.

Tax reform began last spring also as a Reagan initiative, managed by Baker. And in one sense at least it has been a success. Reagan's number-one priority, it seems, is to avoid a tax increase. On the tax bill the danger was that a tax increase would be smuggled in, but Reagan averted that.

Then he failed to advance the Rostenkowski bill when it emerged from the Ways and Means committee. Here the president seems to have been ill-served by bad advice from White House Chief of Staff Donald Regan, who derailed a quick endorsement of the Rostenkowski bill on the grounds that it would irritate House Republicans and prevent them from coming forward with their own alternative.

But the House Republicans couldn't come up with an alternative that could pass the Democratic House. Regan and evidently Reagan also failed to realize that the House Republicans are not the president's natural allies on this issue (or on most others); they are a resentful minority, out of the loop on most issues while other Republicans all over town are running things.

So they jumped at the chance to beat something, even Ronald Reagan -- they voted against him on tax reform and defeated him.

Now they need some cosmetic concession, a fig leaf of some sort, to hide the shame this coming week of backing a president they defeated only days ago.

But Rostenkowski and the Democrats are adamant that they can't have any. In Italy, Craxi gave Giovanni Spadolini a fig leaf so that Spadolini could return to the government after resigning in protest over Craxi's decision to let the mastermind of the Achille Lauro highjacking escape.

Spadolini had no choice but to come back. But House Republicans and House Democrats can allow the tax bill to die without any harm. Even Rostenkowski won't be hurt badly; everyone likes to stay friends with the chairman of Ways and Means. The one hurt is Reagan.

In effect, Reagan chose, knowingly or not, to risk losing tax reform and to embrace Gramm-Rudman. If the former decision hurts his chances to lead a parliamentary government, the latter could turn out to be downright disastrous.

Originally the Senate version of Gramm- Rudman looked good to Reagan, because it promised politically painless domestic spending cuts and no pressure for a tax increase. Donald Regan was quick to back it.

But the Reagan White House and just about everyone else failed to anticipate the brilliant leadership by O'Neill which got just about every House Democrat, from former Boll Weevils to most of the Black Caucus, behind the Democratic alternative.

The House Democrats' version of Gramm-Rudman would kick in with spending cuts well before the 1986 election when 22 Republican senators are up, and mandate deep cuts in defense unless Reagan compromised on taxes or domestic spending with the Democrats.

Now the Senate Republicans and House Democrats have split the differences between them and passed Gramm-Rudman into law. Reagan has endorsed it. But the bill creates new problems that might cause a revival of the grand coalition, still smarting from its defeat.

It's easy to imagine their solutions to the dislocations caused by big budget cuts: a continued slowdown of defense spending; cuts in domestic spending, too, but not a lot; and -- in some form, overt or disguised -- a tax increase. They might even come up with a military draft, which would save a lot of defense money.

Now those are not results desired by Ronald Reagan, and it's possible that he could fight the grand coalition on this, and maybe even win. But he will do so from a position of considerable disadvantage -- one which he need not have been in if he had made different choices these past few weeks.

It should be added that Reagan's problems are by no means as great as those of other landslide presidents who got themselves in trouble. By this time in their second terms Roosevelt had gotten licked on court packing, Johnson was mired in Vietnam, and Nixon was facing impeachment hearings on Watergate. Reagan's biggest political mistake in 1985 was Bitburg which, grotesquely, raised his ratings in the polls.

He can survive continuing controversy over the budget pretty well. He and, as 1984 shows, the voters are willing to endure the admitted evil effects of the deficits in preference to many alternatives.

Moreover, Reagan's lack of the sort of landmark accomplishment that American historians and journalists have come to expect of successful presidents is not fatal. Reagan seeks to produce conditions, not pass laws or sign treaties: facts on the ground, not pieces of paper.

But what Reagan has done, by letting tax reform become so precarious and embracing Gramm-Rudman, is to make it much more difficult for him to sustain this fourth parliamentary government of his presidential term.

He had a chance to do in the United States what Craxi, head of a party that gets 13 percent of the vote on a good day, has done in Italy: change the life of the country by making himself the prime minister, the operative boss of a parliamentary government that controls policy on the lead items on the national agenda. Unless he pulls out tax reform, he will be struggling again and again with the grand coalition leaders and with political freebooters like Gramm over one issue-of-the-week after another, in the style of Italian politicians a decade or two ago.