Like any third party worthy of the name, Britain's Social Democratic Party (SDP) is affecting the two major parties whatever its own political fortunes turn out to be. It is not known whether the SDP will use its second year to build on the victories of its first and become more than a phenomenon of midterm voter dissatisfaction. It ism known that the polarization of the incumbent Conservatives and the opposition Labourites has been muted a bit in their efforts to stem defections to the SDP.
A Conservative budget with increased regard for social security and unemployment. Labour allusions to wage restraint and union efforts to control leftist elements. Would they have come or come as quickly without the SDP? At least they should not be ignored in the reverberations from the SDP's dramatic first anniversary present -- Roy Jenkins's triumph in what British politicians were calling the most important by-election since World War II.
It might have been more decisive if Mr. Jenkins, the candidate of the SDP in its alliance with the long-established Liberal Party, had lost. Such an outcome was expected to accelerate the alliance's decline in the polls after its unexpectedly early peak -Mr. Jenkins's long and distinguished political career. By winning, he opened the possibilities for many more decisions: by the SDP, by its Liberal allies, and by the voters.
SDP and Liberal figures immediately began endorsing Mr. Jenkins for alliance leadership and thus potentially the prime ministership in the event of defeating both the major parties at the next general election. But united support for him was not a foregone conclusion. He is a conservative in relation to David Owen and Shirley Williams, other founders of the Social Democratic Party. There could be a struggle over party leadership when the chips are down on the ''socialist'' -- though not extreme left -- issues many SDP members want to retain from their Labour Party origins. They would see eventual political success more likely with a center-left coalition, considering Labour's weakened state, than a center-right coalition, vulnerable to true-blue Conservatives. They would want to be sure Mr. Jenkins intends to go in their direction.
Winning a parliamentary seat held by Conservatives for six decades may turn out to have been easier for Mr. Jenkins than winning his own party battles. But by keeping to the ideals of progress and moderation the alliance could continue to nudge both major parties toward the center -- an achievement in itself.