AS the glow of last month's victory fades, Republicans might find themselves reflecting with Wellington that ''nothing except a battle lost can be half so melancholy as a battle won.'' The dark side for the GOP is that the party must now govern during four perilous years with a lame duck (and perhaps faltering) President, a hostile House of Representatives, and a Senate majority imperiled in 1986. Tack on an impending struggle over President Reagan's successor and the Republicans will be pressed to retain the White House in 1988, despite sweeping 49 states this fall.
Not since Franklin D. Roosevelt succeeded himself in 1940 has either party been able to secure three consecutive terms in office. The Republicans tried twice and failed, in 1960 and 1976, the Democrats once, in 1968. Each defeat, moreover, followed a landslide victory in which incumbent Presidents (Eisenhower , Johnson, and Nixon) prevailed by an average margin slightly greater than Reagan's lead over Mondale in 1984.
Analysis that foretells presidential results discloses why a third consecutive term will probably elude the Republicans once again in 1988. Scrutiny of every election from 1860 to 1984, using methods refined in earthquake prediction, shows that the answers to 13 questions indicate whether situations prior to an election favor the incumbent or the challenging party.
By probing the political strength of the party in power, economic conditions, policy change, scandal, social unrest, foreign policy successes and failures, and the charisma of competing candidates, these keys to the presidency have correctly indicated the winner in 31 of 33 presidential elections. Issues, ideology, party affiliation, strategy, and tactics have mattered only to the extent that they influence key indicators.
To be confident of victory in an upcoming campaign, the party in power needs to secure in its favor nine or more of these 13 indicators of incumbent party success. A gain of seven or fewer keys portends defeat and a gain of eight keys a tossup election.
With 11 of 13 keys turned in his favor, Ronald Reagan was well positioned for reelection in 1984. Only a lag in annual per-capita growth (remember 1981 and 1982) and the lack of a foreign policy triumph counted against the party in power this year.
Yet the historical odds may already be turning against the GOP just as they did following Eisenhower's reelection in 1956. Among the advantages that helped Reagan roll to victory were incumbency, an uncontested nomination, policy innovation, his own charisma, and the lack of an inspiring opponent. All are in jeopardy four years hence.
The near extinction of liberal Republicans has not united the party, only shifted rivalries to the right. Without Reagan to pull Republicans together the question is when will the GOP's moderate, conservative, and radical right factions begin squabbling over the post-Reagan succession. Conservatives fired the first shot when Rep. Newt Gingrich (R) of Georgia warned that any attempt to compromise with Tip O'Neill's liberal gang, would ''split the GOP by Easter and mark Reagan as the William Howard Taft of the late 20th century.'' But moderates may have won the first battle with the selection of Robert Dole of Kansas, a self-styled ''moderate-conservative,'' as Senate majority leader.
Factional strife guarantees a contested nomination in 1988 and weakens the ability of a lame-duck administration to effect policy innovations comparable to those of Reagan's first two years. The Republicans, moreover, can scarcely anticipate that their struggles will yield a nominee with the President's personal appeal while the Democrats may search for a charismatic challenger (Mario Cuomo? Ted Kennedy? Gary Hart?) unsullied by the Carter connection.
Under these circumstances the Republicans could not withstand any of the usual mishaps of a party's second term. To sustain hopes for 1988, the administration would have to keep the economy humming, maintain tranquility at home, avoid disaster abroad, avert damaging scandal, and score a major foreign policy success.
Barred from salvation through the Roosevelt expedient of self-succession, the Republicans might ironically find that their brightest scenario for 1988 includes an uncompleted Reagan term and the ascendency of George Bush to the presidency. They could thereby gain an incumbent nominee and perhaps even avert an intraparty battle if the Republican right reconciles itelf to Bush. New leadership at the top might also enhance prospects for major policy change or a triumph in foreign affairs.
''Finality,'' Disraeli observed, ''is not within the language of politics.'' Twice buried by the great campaigner, Democrats may now find circumstances favoring a comeback next time around. Yet beyond the choice of a charismatic nominee, challengers can influence their own fate by exposing failures of the opposition regime. Thus the verdict of history in 1988 may as yet turn on the quality of statecraft in Ronald Reagan's second term.