LONDON -- Many Americans, disgusted by presidential campaigns that are coarse and seem interminable, envy what they imagine to be the civility and brevity of British campaigns. However, one experienced and therefore unenthralled Laborite member of Parliament predicts that Britain's next campaign, currently getting into gear at least one and perhaps two years before the voting, will have two nasty themes: ''She's a bitch'' and ''They're liars.''

The formal British campaigns of a few weeks' duration are just the final sprint at the end of a marathon. Margaret Thatcher's government, which can call the next election to suit its convenience, is already toiling to produce convenient facts, particularly sharply lower interest rates.

Pursuant to her goal of producing property-owning democracy (as nationalized industries were privatized, the number of persons owning shares of corporations passed the number who are members of labor unions), Thatcher encouraged the private purchases of publicly owned houses. More than 1 million were bought. But many homeowners have variable-rate mortgages that originally were at 8 percent and now are at 15 percent.

One reason for the rise of interest rates is German reunification and the surge of demand for capital there and in Eastern Europe. The political ball takes strange bounces: the fall of the Berlin Wall could, or so Labor hopes, lead to the fall of Europe's foremost conservative government.

Labor, having issued a new manifesto of impeccable vacuity and brazen evasions, has completed the task of emptying itself of traditional conventions. The prime minister, the model of a ''conviction politician,'' has many dogmas and a manner of propounding them that is, to many, maddeningly familiar. Labor has now committed apostasy against every familiar tenet of its ancient faith (socialism -- the unutterable S-word), so this campaign is not going to be pretty.

In democracies, the bitterness of politics often is inversely proportional to the doctrinal differences between the parties. A common acronym here -- TBW, That Bloody Woman -- recalls Republican asperity referring to Franklin Roosevelt as That Man in the White House. Labor, having jettisoned convictions, will count on subordinating political judgments to aesthetic judgments. Labor will play to the antagonism many voters feel toward Thatcher's persona as national nanny.

She has, says Francois Mitterrand, the eyes of Caligula and the mouth of Marilyn Monroe. But discontent with manner may be a weak lever with which to pry this incumbent from power. To the extent that style is the issue, substance is not. It is not because Thatcher is a success, a fact to which Labor's apostasy pays tribute. She set out 11 years ago to destroy socialism and make Britain more wealthy and productive. She did.

However, successful political parties often are undone by their successes. America's Democrats used activist government to create (through subsidized college education, home mortgages and the infrastructure of suburbanization) a middle-class majority which, in time, decided it could do with less government activism. One reason Britain's Conservative Party is vulnerable is this: upwards of 40 percent of the voters in the coming election were not of voting age in 1979, when Thatcher won in ''the winter of discontent.'' That trauma, when even hospital aides and gravediggers were on strike, and unions rather than government seemed sovereign, now seems as distant as Agincourt.

In the 1983 election, Labor lurched left, producing a platform described as the longest suicide note in history. But now Labor has forsworn unilateral nuclear disarmament, confiscatory taxation (the top rate might rise from 40 to 50 percent, but just 11 years ago was 98 percent), nationalization of the economy's ''commanding heights'' and reestablishment of untrammeled union power. It even makes rhetorical bows to the market, and this intellectual sunburst: before wealth can be distributed, it must be created.

By calling Labor liars, Conservatives are poised to have it both ways. They will say that Labor, having at long last abandoned silly socialist principles, is contemptible because it is unprincipled. Or if not that, then it is mendacious: Labor does not mean it. Conservatives will say that principles so casually tossed in the dust can be casually plucked up again.

Thatcher, now in her 12th year at No. 10 Downing Street, is vulnerable to the volatility that boredom breeds. And another threat to her is the British appetite for political fudge -- the preference for soft options, blurred choices, mushy compromises, muddling through. Still, the stricken field of British politics is littered with those, Tory as well as socialist, who underestimated Thatcher. The smart money says nanny wins a fourth term.