As election dust settles, tests of policy return to center stage; THE ECONOMY

If one thing, more than any other carried President Reagan to his landslide victory, it was probably was the ''pocketbook factor'' - the widespread sense of economic well-being throughout the United States on election day.

The biggest component of the pocketbook factor: lower inflation, thus more buying power.

True, interest rates were still high. But only borrowers are affected by these, and borrowing has never been universally regarded as wholesome in America. The budget deficit was a big worry - but this problem seemed intangible to many voters. And although unemployment was still considerable, thousands of new jobs had been created in Mr. Reagan's first term.

The pocketbook was not the only factor in the Reagan win. A month before the election, a Citicorp analysis of the ''misery index'' (unemployment plus inflation) noted that ''if gut economic issues were all that this election is about, the Reagan-Bush ticket would win with about 51 percent of the popular vote.'' At 59 percent, Reagan got pocketbook-plus.

What of America's economic future?

Prosperity should stay on track at least during the opening months of the second Reagan term, most economists agree. The consensus for late 1984 and early '85: Expect low inflation and falling interest rates, but beware of an economic slowdown, maybe even a recession, next year. Further ahead in the Reagan years, however, those intangible federal deficits may get worse, and a weakening dollar could reignite inflation.

''The economy is slowing down,'' says Martin Feldstein, the most recent chairman of Reagan's Council of Economic Advisers (CEA) and now back at Harvard University. ''People will have to face up to the fact that we're not going to grow our way out'' of the deficit problem.

Tax reform, many agree, will be widely discussed in the coming weeks and months. But a major revamping may elude Washington. There are just too many constituencies that would be hurt by an across-the-board flat tax (a simpler tax that eliminates most deductions and charges virtually the same rate to all taxpayers in an income group).

Nearer at hand, a slide in interest rates appears to be encouraging America's financial markets even more than the prospect of four more years of the Reagan administration. One indication of that slide appeared Wednesday, when several major US banks lowered their prime rates from the prevailing 12 percent to 11.75 percent.

On election eve, the Dow Jones industrial average jumped 12.59 points; on election day it rose 14.91 points. It was not doing so well at this writing Wednesday, however.

The markets' strength appears linked more to receding interest rates (stocks look better when money-market rates fall) than to a perception of better days for corporate America under a reelected Reagan. In fact, in the near term, many economists say, the US appears in for a slowdown, if not a mild recession. Neither would be great for corporate America's profits.

Murray L. Weidenbaum, who was chairman of the CEA early in Reagan's first term and is now at Washington University in St. Louis, contends that ''1985 will not be as good as '84.''

A slowdown is already apparent, he says, and economic growth in '85 will be only half that of '84.

''But a Mondale win would have made no difference,'' Dr. Weidenbaum adds. The slowdown was inevitable, and ''whoever is out of office can blame the other.''

The US economy ebbs and flows in a way that has always defied the collective control of the president, Congress, and the Federal Reserve Board - the key economic decisionmakers. The Fed appears likely to try to bolster the economy by loosening the money supply. But the two-year-old economic recovery/expansion may be getting tired on its own accord today, economists say.

As the US economy eases back in the opening scenes of Reagan II, the rest of the world economy may improve slowly or not at all. Countries that rely on commodity exports (oil, cocoa, and copper, for example) have been facing chronically weak prices; recent attempts by the Organization of Petroleum Exporting Countries to hold oil prices at their current level is one prominent example of this.

Thus many oil and nonoil countries have continuing debt-repayment worries. Some countries will fare better under lower energy prices; others will not.

Lower US interest rates would help many nations. Lower rates would cause the dollar to weaken against foreign currencies (as it has been in recent days), and that would keep foreign capital in home countries for home-grown development. But foreign goods could become less competitive with US goods, and that would hurt other nations somewhat.

''The dollar has dropped sharply because of the lower interest rates,'' says David A. Wyss, an economist at Data Resources Inc. in Lexington, Mass. ''At present we believe the European countries will cut interest rates, cushioning the slide in the dollar.

''But a sharper fall is a very legitimate worry, since the speculative money that has been coming into dollars could flee very quickly.''

What about those US budget deficits? Neither economists on the right nor on the left can see a way to trim the deficits, especially considering the balance of power between a GOP White House and Senate and a Democratic House.

On the one hand, Reagan has promised not to raise taxes. As economist Paul L. Samuelson of the Massachusetts Institute of Technology notes, ''The President simply hates taxes.'' Dr. Feldstein observes, ''He hates high tax rates.''

With the tax take unlikely to grow, Congress will again be faced with cutting the budget. Republicans in Congress and in the White House, Dr. Samuelson says, will seek to cut social spending, specifically entitlement programs, and Democrats will seek to trim defense.

''There will have to be more emphasis on entitlements,'' Feldstein agrees.

Weidenbaum says he thinks the demand to raise revenues will be so great that it's possible that a cap on interest deductions on home mortgages or an elimination of the investment tax credit for business could emerge next year. ''The choice will be whether to sock it to the consumer or business,'' he says.

But all three economists indicate it's unlikely the deficit will be reduced forthwith. If an economic slowdown hits, the deficit may grow, because government must spend more to cushion Americans.

In an attempt to tame the deficit, Weidenbaum reckons Republicans will try to impose spending controls via a constitutional amendment requiring a balanced budget. Samuelson agrees this will be a big push: ''If I were a conservative, I'd be thirsting for it.''