The lesson of the Senate's rejection of John G. Tower as secretary of defense is that President Bush should never have picked so slapdash a Cabinet - old pals, multimillionaires and textbook conflict-of-interest cases - after winning such a sour presidential campaign with so limited a mandate. The And this may only be the beginning. The administration seems to be stumbling into a four-year dogfight with Congress.Trivialization is the name of the game when a Democratic party that elects Edward M. Kennedy, D-Mass., to be chairman of the Senate Education and Labor Committee spends day after day belaboring Tower's reputation for chasing skirts and popping corks.

In a sense, the Tower debate underscores how little Washington's Republicans and Democrats stand for.

If either side has a meaningful agenda for this country it's well hidden among the scandals and global junkets.

Without some major mood-shift, the next four years could be mean. Federal judicial confirmations could turn into latter-day Spanish Inquisitions.

The chairmen of the Republican National Committee and the National Republican Congressional Committee have served notice that Democratic congressional incumbents can expect personal attacks in 1990.

Errant senior Bush administration officials, in turn, will have to worry about special prosecutors. And when the long-awaited committee report on Democratic House Speaker Jim Wright's ethics problems comes out, Wright can look for the same careful, bipartisan consideration meted out to Tower.

Republicans, meanwhile, have been fooling themselves by trumpeting Bush's prerogative to pick anyone he wants for his Cabinet. That's unrealistic.

Back in 1977, Tower himself cited the Senate's advise-and-consent function in unsuccessfully opposing President Carter's selection of Paul C. Warnke as arms control and disarmament adviser. One could also note John F. Kennedy's caution in 1961, after narrowly winning the White House in an election where the opposition scored simultaneous gains in Congress and the states.

Well aware how much this limited any "mandate," Kennedy picked two Republicans for his Cabinet.

By contrast, Bush picked most of his top Cabinet secre taries and appointees from a clique of personal political supporters.

No other late 20th-century Cabinet so closely parallels the senior hierarchy of previous administrations. Capitol Hill Democrats had some reason to feel provoked.

Provocative circumstance No. 2 lies in how Tower represents just the tip of the iceberg with respect to Bush administration conflict of interest.

The new president's condemnations of the middle-class hustler ethics surrounding Reagan era officials like Edwin Meese III and Michael K. Deaver have an element of both inadequacy and class condescension.

Top-level Bush appointees are so much richer - multimillionaires Baker, Brady and Mosbacher have a combined net worth of about $250 million - that they have an entirely different sort of conflict-of-interest problems.

Baker's situation, in fact, may be the administration's most egregious. We know now that while Baker was secretary of the Treasury, he owned several million dollars worth of stock in New York's Chemical Bank - but nonetheless pursued Latin American debt-related policies.

Former Reagan White House Communications Director Patrick J. Buchanan has suggested that Meese would have gone to jail for what Baker has done, and the New Republic has pointed out that while Baker was busy favoring the interests of banks like Chemical, his own stock holdings appreciated nicely.

Tower's situation, by contrast, had he become secretary of defense, would have meant recusing himself from most major Pentagon decisions because of his 1986-88 service as a defense industry consultant.

This was what the public found most objectionable about Tower's nomination, yet other Bush Cabinet officers have at least related situations.

It says little for the quality and depth of the Democrats' fight against Tower that they kept emphasizing the hapless Texan's personal behavior while avoiding serious discussions of Tower's conflicts-of-interest - or the larger problems in the Bush administration as a whole.

Nor did they choose to focus on Bush's emerging judgment shortfall in choosing inadequate aides, advisers and vice president. Yet these are important issues.

Now that we've seen Tower come undone, Bush's selection of Dan Quayle for vice president is looking less like a fluke and more like a harbinger of recurrent ineptitude.

But whatever Bush's problems, he does have one break: the acumen of the Democratic opposition. It is the public that has bipartisan cause for complaint.

(Kevin Phillips is publisher of the American Political Report and Business and Public Affairs Fortnightly.)