In a year, the presidential campaign will be entering its final, thunderous phase. The Republican hopefuls -- Vice President George Bush, Sen. Robert J. Dole and all the others -- must now be wondering: can the economy's strong performance continue that long? Democrats are surely asking the same question. The economy now belongs to the Republicans. Unless things change, the Democrats may need a miracle to recapture the White House.

Oceans of political commentary will flow between now and then. Issues and tactics will be analyzed. Much of this outpouring will be irrelevant. A presidential election is not usually a careful choice between opposing political ideologies. Nor do political strategists typically decide the outcome. Rather, a presidential election is a crude public referendum on the present and recent past. Prosperity now favors the incumbents.

There is nothing subtle about this. Since World War II, only unpopular wars and weak economies have consistently forced incumbent parties from the White House. The Korean and Vietnam wars cost the Democrats in 1952 and 1968. In 1960, Richard Nixon lost narrowly to John Kennedy; the economy was in recession. When Jimmy Carter beat Gerald Ford in 1976, unemployment remained high from the 1974-75 recession. Double-digit inflation was Jimmy Carter's chief liability against Ronald Reagan in 1980.

Of course, more than pocketbook issues matter in politics. But they are critical for middle-of-the-road voters. When there's a general sense of well-being, people are less eager for change. Other issues have less of an impact. Thoughtful Democrats know this and must have silently cheered last week when the Federal Reserve raised its discount rate -- the interest rate charged by the Fed on its loans -- from 5.5 to 6.0 percent. Higher interest rates could mean that the economic recovery is finally fraying at the edges.

Time will tell. But for the moment, the recovery endures. Commentators have termed the economy's performance this year "moderate" or "sluggish." One network television reporter recently called it "creaky." Language obscures reality. The recovery that began in late 1982 is now entering (in September) its 58th month. Of the nine postwar economic recoveries, only one -- the 106-month recovery between 196l and 1969 -- has lasted longer, and arguably it was sustained by Vietnam war spending.

No one should expect rapid, spectacular economic growth in the fifth year of a recovery. What's desirable is steady expansion that's adequate to absorb the rise in the work force and to create higher living standards. That's precisely what the economy seems to be producing. Consider:

Modest growth continues. For 1987, most economists expect an increase of about 2.5 percent in the gross national product. That's the average forecast from the 51 economists surveyed by the Blue Chip Economic Indicators. Their average estimate for 1988 is 2.9 percent.

Over the past year, the number of jobs has risen 3.2 million. The civilian unemployment rate has fallen from 7 percent in July 1986 to 6 percent in August 1987. Since the recovery's start, the number of jobs has grown 14 million.

Living standards are rising faster than in the late 1970s, though increases are modest. The same can be said of productivity growth -- the source of higher living standards. In the current recovery, businesses' output per hour has increased 1.9 percent annually. That's less than 2.6 percent average for all postwar recoveries, but better than the 1.3 percent of 1975-80.

The trade deficit is turning around. Export volumes are up, while import volumes are down. The reported trade deficit, which hasn't dropped, obscures the shift. Because a depreciating dollar means that imports cost more, the lower volume of imports has had a higher price tag. Adjusted for price changes, the trade deficit has declined about 18 percent since the summer of 1986.

Democrats cannot draw much satisfaction from this overview. Of course, huge problems persist. Both the trade and budget deficits remain immense. Also, the recovery has been uneven. Parts of the Farm Belt and Oil Patch are still depressed. The proportion of Americans below the official poverty line hasn't dropped much. But converting these problems into potent campaign issues requires some economic turbulence.

Otherwise, criticisms -- no matter how compelling -- are neutralized by the general prosperity. Voters are more impressed with tangible, present successes than with future, possible problems. "Most Americans believe the country is on the right track, and they are optimistic about the economy," writes opinion analyst William Schneider in the National Journal. In that climate, other issues affect their outlook only "on the margins."

A cynic must wonder: have the Republicans cooked the economy for the election? A few years ago, here's what a Republican strategist might have advised the White House to do: (1) Promote a dollar depreciation to make U.S. exports more competitive. (2) Nudge out Federal Reserve chairman Paul Volcker. His anti-inflation zeal might make him too eager to restrain the economy. (3) Ignore the budget deficit. Despite the long-term benefits of smaller deficits, higher taxes and lower spending might initially hurt the economy.

In a nutshell, that's been the administration's economic policy. Is it mere coincidence or shrewd strategy? Whatever the truth, both Republican prospects and the recovery may now be riding on borrowed time. No recovery lasts forever. This one could end in numerous ways. Inflation could accelerate, in part because a depreciating dollar raises the prices of imports. Debt-laiden consumers could further slow their spending, while feeble foreign economies fail to provide an offsetting stimulus to U.S. exports.

What ought to worry Republicans -- and hearten Democrats -- is that the Republican economy may be too good to be true. Politically, it may be running ahead of schedule. In September l987, it may have achieved the dull, reassuring prosperity that Republicans had envisioned for November 1988. Political handicappers stay tuned: the White House may hang in the balance