Declining financial markets have been sending President-elect Bush a pointed message lately. The message: The financial community has little or no confidence that a $150 billion federal deficit can be eliminated in four years with no new taxes.

It's a message that Bush had better take seriously since it is coming not just from investors but from a wide variety of sources. Only this week the General Accounting Office reiterated its long-standing warning that Bush's flexible-freeze program of holding federal spending increases to the rate of inflation is unworkable. Many private economists feel the same way. The GAO, a financial arm of Congress, is calling for increased taxes.The same points were made the other day by former presidents Ford and Carter, who suggested increases in taxes on gasoline, tobacco, beer, and wine together with a limit on increases in Social Security benefits.

It would be a mistake to think that Bush's response to these messages consists only of his stated reluctance to break his campaign promise to avoid new tax increases.

Part of the response also consists of the shy flirtation he is known to be conducting with the possibility of raising new revenue through a wide variety of user fees. Though such fees would hurt some Americans, it's only fair that those who utilize such things as outdoor recreation facilities foot the full bill for them. But such fees would make only a dent in the deficit.

Another part of Bush's response can be seen in some of the president-elect's initial choices for his new Cabinet. That goes particularly for the selection of Nicholas Brady as Treasury Secretary and Richard G. Darman as budget director.

Both Brady and Darman are moderate conservatives. Likewise, both are practical men, more interested in solving problems than in pursuing ideological purity.

Darman, who has an impressive background in both government and private investment banking and has been a master at forging consensus between Republicans and Democrats, has the unenviable challenge of leading the fight to bring down the federal deficit. Even though he is a workaholic and a perfectionist, Darman is bound to be frustrated unless the new administration realizes there are sharp limits to how much it can reduce the deficit with spending cuts alone. The frustrations are bound to be especially acute unless Bush abandons President Reagan's old policy of trying to put the military off limits to budget cutters.

The simple fact is that three-fourths of the federal budget consists of interest on the national debt, Social Security, and defense expenditures. The rest of the budget is made of up programs that already have been cut, in some cases cut repeatedly.

Though Social Security recipients should expect to share in the belt-tightening, few elected officials are brave enough to limit their benefits, knowing that even a suggestion along those lines risks early retirement from politics.

Higher gasoline taxes would not only raise substantial amounts of revenue, but also help eliminate air pollution by discouraging unnecessary driving. The trouble is that such taxes would hurt the poor the most and would stimulate inflation by adding to the price of all goods and services distributed by trucks and planes.

But it would be a mistake for Bush to rule out all tax hikes, such as those being suggested for tobacco, wine, and beer. The proposed new tax on wine and beer would simply hike that levy to the same level as the one on hard liquor. Likewise, a 25 cents a pack tax increase for tobacco would not only raise $21.8 billion over five years, but also might help keep teenagers from getting hooked on smoking. Less smoking, in turn, would mean future savings on medical bills.

Speaking of medical bills, the government's medical care costs have been rising at two to three times the general rate of inflation. This is one area where federal spending simply must be curtailed.

But the biggest opportunity for savings is to be found in the bloated Pentagon budget, which in terms of constant dollars consumes more money than at any time since World War II - even more than at the height of the wars in Korea and Vietnam.

If Bush abandoned Reagan administration plans for a 4 percent hike in the defense budget to keep up with inflation, he could save $255 billion over the next four years. This could be done either by sharply slicing major weapons programs now - or by stretching out production schedules and making a variety of smaller slices.

In any event, the new president has no realistic choice: Either he makes meaningful progress toward eliminating the deficit or a nasty chain reaction is likely to set in. Eventually, worsening investor confidence is bound to rub off on the economy. And a sour economy would swell, rather than shrink, the deficit. There are worse things than a few, limited tax hikes plus belt-tightening at the Pentagon.