London — These are difficult days for Britain's battered political left. The center right Conservative Party is firmly reestablished in power in Parliament, with a massive majority. The old socialist language of trade unions and the Labour Party seems at odds with the decline of traditional, highly unionized industries, and with modern lives and needs.
Now the left faces yet further setbacks: widespread public upset over the Soviet Union's shooting down of Korean Air Lines Flight 7 and the Kremlin's handling of it; and reinforced suspicion of Moscow flowing from West Europe's expulsion of a mounting number of Soviet diplomats accused of spying.
Tory ministers are convinced that anti-Soviet feeling in recent months has made it more difficult for trade unions, the Labour Party left, and nuclear peace protesters to argue positions that coincide with policies followed by the Kremlin - or that seem to the public to coincide with them.
The left wing itself, in all its varying shades, rejects this view. But:
* The Trades Union Congress (TUC) swung to the right at its recent annual conference in Blackpool after a mere 39 percent of unionists had voted for the Labour Party in the June 9 general election. Left-wingers, with the general mood already against them, found the Korean airline disaster dominating the news and causing many British people to look with even more distaste on ideas they associate, rightly or wrongly, with the far left and communists in general.
* Far-left-winger Arthur Scargill, leader of the National Union of Miners, ran into strong criticism at the TUC conference and from others in the Labour Party, for supporting Soviet disarmament initiatives and criticizing the Solidarity union in Poland. Both of ''King Arthur'' Scargill's pro-Soviet views would have drawn criticism in any event. But the loss of the Korean plane intensified it and led to even more widespread condemnation in the press.
* The nuclear protest movement is reduced to ''symbolic'' protests against the new NATO cruise missiles that are now virtually certain to be installed in Britain by year's end. The movement faces the feeling voiced by many British people these days that any disarmament desired by the country that shoots down civilian airliners must be bad.
* The Labour Party itself is still torn by battles between its far left (mainly Trotskyites) and its more moderate center. Traditional moderates, such as economic spokesman Peter Shore, have seized the opportunity to denounce Mr. Scargill for his views.
The Labour Party is not only reduced to 207 seats in the 650-seat House of Commons, but also has just reported its lowest grass-roots membership since World War II (273,803), and a bank overdraft of (STR)525,000 (about $787,000).
Party general secretary James Mortimer is said to have concluded in a report on why Labour lost so badly June 9 that the party was simply out of step with the people in advocating unilateral disarmament, pulling out of the European Community, and rejecting Conservative plans for union reform.
Mr. Scargill himself says he is deeply hurt by the loss of civilian lives on the Korean airliner. He refuses, however, to retract remarks he made in Moscow just before the event, in which he strongly attacked ''President ray-gun'' and ''plutonium blonde'' Margaret Thatcher while praising Soviet disarmament proposals.
One case for the left is put by a national organizer for the Campaign for Nuclear Disarmament (CND), Dave Wainwright. In an interview, he agreed that some Britons might now feel more inclined to oppose CND's anti-cruise-missile campaign because the Soviets are against the cruise as well.
''But,'' he says, ''we believe that the incident, with its tragic loss of life, only proves how vital it is to stop nuclear weapons now. If the two superpowers, with their hot line, can't sort out the fate of a single airliner, how can they manage weapons of mass destruction?''
It would certainly be wrong to dismiss the British far left. Left-winger Ray Buckton of the Train Drivers Union is the new president of the Trades Union Congress. Leftist Neil Kinnock is heavily favored to succeed left-winger Michael Foot as leader of the Labour Party later this year.
Union membership is down from more than 12 million four years ago to around 10 million today, which is only about half of the British work force, and Prime Minister Thatcher is talking about not one but several union reform bills in the Commons.
But union power still cannot be discounted.
The influential Economist magazine concludes that unions have passed the peak of their power, but adds that an obituary is premature. Unions, it says, ''will be a force in the land for years to come and in some industries a formidable force.''
But in a number of areas the left is struggling, and Soviet discomfiture only makes its task more difficult.