JUST when opposition leader Neil Kinnock seemed to have tamed troublemaking radicals in his Labour Party, a threat has arisen to challenge his authority and threaten the party's long-term electoral prospects in Britain. It is coming from Trotskyite activists dedicated to infiltrating the party at the political grass roots.
Out of office for more than a decade, Labour lately has begun to ride high in the public opinion polls and is putting heavy pressure on Prime Minister Margaret Thatcher, whose personal standing has been declining.
Now however Mr. Kinnock's strategy of promoting Labour as a party of the moderate left, capable of winning back the hearts and minds of millions of voters, is endangered by the actions of activists in a constituency party in Liverpool.
The local party organization on Dec. 8 voted to drop Frank Field, Labour's greatly respected member of Parliament (MP) for the city's Birkenhead seat since 1979, and replace him as a candidate at the next general election with a little-known Liverpool trade union official.
Despite Mr. Field's large parliamentary majority in the 1987 general election, he found himself ousted by activists, many of them members of Militant Tendency (MT) - a Trotskyite group that for the past 10 years has been trying to get a grip on Labour's political machine in many parts of the country.
When the result was declared, Field appealed to Kinnock to order it set aside. He threatened to resign his seat and force a by-election if the Labour leader failed to act. Field claimed that a high proportion of the votes against him had been from MT members.
Kinnock responded by ordering an inquiry into the Birkenhead result. Field then withdrew his by-election threat.
In a by-election, he would stand a good chance of winning as an independent Labour candidate. But such a battle would sorely expose Labour's internal divisions and reduce its credibility as a vote winner in a general election.
Field, an unashamed intellectual, has strong sympathies for the poor. He is chairman of the House of Commons social services committee - an unusual distinction for a Labour MP during a period of Conservative government.
``Frank is a maverick in some ways,'' says a Labour Party official in Liverpool. ``But as a former director of the Child Poverty Action Group, he brings unrivaled authority to his work. We can't afford to lose an MP like him.''
Field's ``maverick'' reputation (he supports market forces and has been known to approve some Thatcherite policies) did not help him in the Birkenhead vote.
As Labour's national leader, Kinnock has power to quash the result of the Dec. 8 vote in Liverpool, but only if he finds evidence of malpractice. Also he must take into account that the bulk of Labour Party finances come from affiliated trade unions.