All things being relative, from a European perspective, President Reagan has good reason to relish his re-inauguration. Only British Prime Minister Margaret Thatcher, among the leaders of the Western Alliance, has as firm a grip on four more years. That's the up side for a second- term American president with an eye to history and to a lasting imprint on peace in his time.

For this, he will obviously be at the mercy of the coincidence of Soviet interests with his own. But Soviet interests, in turn, will be shaped by the Kremlin's perception of the ebbs and flows of political tendencies in Western Europe, where the East-West confrontation is most acute. A coming together by happenstance of European election timetables combines with the slide from grace of incumbent government heads and/or ruling political parties and coalitions to produce the prospect of a collective condition: intensifying and debilitating public debate, with a potential for more than the usual ferment and discontinuity.

If one-upmanship was all there was to it, you would have little more than an easy answer to endless European talk about the impossibility of following American leadership when it keeps changing all the time. They should keep their peace -- given their own swings from left to right or the other way around in recent years and the likelihood of more to come.

But the more serious message from the collective European political condition has to do with the renewal of across-the-board arms negotiations between the United States and the Soviet Union and the heavy handicap imposed upon Western societies in their dealings with a closed communist system not subject to the fickle fidelity and changing commands of public opinion.

Even those Europeans who were loudly applauding a second Reagan term are beginning to return to worry- as-usual about the continuity of U.S. policy. The European press has been quick to pick up on the implications of the Reagan administration's job changes and departures and the oozing away of power from a lame-duck president. His own party, it's been duly noted, is already looking ahead to congressional elections in 1986 and beyond. As a result, support grows for treating the budget deficit by cutting defense spending in the interest of sparing popular domestic programs.

Leaving aside the direct effect this could have on U.S. bargaining positions on arms control, there could be indirect effects as well. If the United States winds up doing less than expected in its own defense, it will be in a poorer position to press for burden-sharing by European countries whose unemployment averages over 11 percent. At this point, a chain reaction sets in.

European backsliding, if that's how it turns out, works to support any renewed effort in Congress to begin cutting back American troop levels in Europe by way of forcing the alliance partners to shape up. The politicians at NATO headquarters here insist that the threat of U.S. troop withdrawals would have the opposite effect of encouraging latent neutralist inclinations to give up.

Resisting this sort of retrogressive transatlantic interaction would require strong leadership all around. But that is not the way the European political stage is set.

Recent British polls show a four- point drop in approval for the Thatcher government, to 40 percent -- a reminder that under Britain's particular electoral rule the great Thatcher reelection landslide two years ago produced a mammoth Conservative majority in the House of Commons out of all proportion to the Conservatives' 43 percent of the vote. That support for the Thatcher government has held relatively steady at that level since 1983 is much less a measure of her sure hand than it is of the almost limitless capacity of her splintered opposition to self-destruct.

In France, President Mitterrand's socialist government is barely ambulatory, with a 30 percent approval rating, and could become a basket case 15 months from now if it loses control in parliamentary elections early next year. And with elections only a couple of years away, the limp leadership of West Germany's Chancellor Helmut Kohl is already vulnerable to economic unrest, the rise of the radical Greens and a Watergate- style scandal that could reach high up into his government.

Normally Belgium might not rate mention on the critical list. But Belgian elections are due before the end of this year, and already the dominant issue has become Belgium's adherence to its commitment to allow the deployment of 48 U.S. cruise missiles in the absence of a U.S.-Soviet agreement to control the numbers of intermediate- range nuclear forces on both sides. If Belgium reneges, it could reinforce second thoughts in the Netherlands.

Checking off these European political timetables over the next four years, it occurs to more than a few European analysts that the Soviets, slow enough by nature to move on arms negotiations, will have all the more incentive to hang tough and bide their time. Allied divisions and dissents, they could reasonably hope, are more likely than not to build pressure for U.S. concessions as time runs out on Ronald Reagan's rendezvous with destiny.