In the light of last week's definitive census returns, Republicans might well paraphrase the Duke of Gloucester: Now is the winter of our discontent made glorious summer by the sun of California. Population shifts provide the only good news the GOP has seen in quite some time.

Under the reapportionment announced last week, California will pick up an additional seven seats in the House. Florida gains four and Texas three. North Carolina, Georgia, Virginia and Arizona gain one each. In recent presidential elections, these seven states have all gone nicely Republican.The census gives, and the census takes away. New York, Pennsylvania, Illinois, Massachusetts, Iowa and West Virginia - states that are not nearly so reliable - collectively will lose 10 seats.

For the Republican Party, the figures add up quite prettily. Because of the reapportionment, the seven gaining states will go into the 1992 presidential election with 159 electoral votes. It takes 270 to win.

If the GOP could hold the 159, only 111 additional electoral votes would have to be garnered. The states of South Carolina, Alabama, Mississippi, Louisiana and Oklahoma might provide 41. Look to the West and Midwest: Ten states that went comfortably for George Bush in 1988 could be counted on for another 47. On these assumptions, the Republican nominee would need to carry only Pennsylvania to be home free.

This is the rosy scenario. Examined a bit more skeptically, and viewed in the light of other recent events, the outlook for the Grand Old Party is not so giddily optimistic. Bush's political future hangs on two uncertain wires: the economy and the Persian Gulf.

Nothing good can be said of the economic prospects. The R-word no longer can be evaded: We are in a recession. Almost all the key economic indicators are down; unemployment is rising. For many retailers Christmas was a disaster. The freeze in California means that the consumer price index will go up an extra point. Even if interest rates drop significantly, the recession may not bottom out until autumn.

No one knows about the Persian Gulf. If Bush wins most of his political aims without a bloody war, he will be a shoo-in for re-election. In the sighs of national relief, the unfulfilled aims will be ignored.

Bush's difficulties are multiplied by his own concessions. His midsummer flip-flop on additional taxes has cost him more heavily than many observers prophesied. Simmering resentment boiled over on Dec. 18 when Pete du Pont, former governor of Delaware, formed a committee "to energize the national will against tax increases."

The only political consolation the Republicans might find lies in the observation that the Democrats are equally in disarray. Apart from Gov. Hamlet of New York, the Democrats have no candidate with a prayer of defeating Bush. They are swiveling over the gulf. The Democrats are doing fine in the state legislatures, and they still comfortably control the two houses of Congress, but the presidency is the big prize. Thanks to reapportionment, that prize has slipped a little further away.