By Keith Weir

LONDON, Sept 10 (Reuters) - Britain's trades unions willlaunch a campaign next week to persuade voters to oppose massivepublic spending cuts, although commentators say their power tosway the government is limited.

The Conservative-Liberal Democrat coalition, which tookpower in May, plans to cut spending by 25 percent in manydepartments to tame a budget deficit totalling 11 percent ofnational output.

The unions, which hold their annual conference in thenorthern city of Manchester next week, fear the cuts will meanhundreds of thousands of job losses in the public sector.

They hope to start to build momentum towards a mass rallyoutside parliament in London on Oct. 19, the eve of theannouncement of the spending programme for the next four years.

Unions are flexing their muscles across Europe asgovernments seek to restore order to public finances followingthe ravages of the global credit crisis, but have had littlesuccess so far in resisting austerity measures. Brendan Barber, general secretary of the Trades UnionCongress (TUC), has drawn parallels with protests in 1990against the "poll tax" -- a local levy that sparked riots inLondon and was eventually replaced.

"The poll tax was defeated when government MPs returned toWestminster to report that their constituencies were in revolt.The poll tax offended the British people's basic sense of what'sfair. So will the spending cuts," Barber said this week.

"Every coalition MP with a small majority and everycoalition MP who fought an election to oppose deep early cutsneeds to feel the pressure from their constituents to changecourse," he added.

The TUC argues that cutting spending too quickly will plungethe country back into recession.

Its delegates will get an expert view on the economicsituation because the governor of the Bank of England, MervynKing, will deliver a keynote speech on Wednesday, filling a slotusually reserved for a senior government figure.

GOVERNMENT NOT SEEKING FIGHT

Union membership has halved since the end of the 1970s butthe TUC still has 6.5 million members, many of them in thepublic sector.

Business Secretary Vince Cable has said that the governmentis not looking for confrontation with the unions and warned themthat hardline tactics could backfire.

He cited the example of the 1978-79 "Winter of Discontent"when unions staged widespread stoppages, helping to bringMargaret Thatcher's Conservatives to power.

"It also undermined public support for the trade unionmovement and opened the way to the Thatcher reforms of tradeunions, which greatly weakened them," Cable told the NewStatesman this week.

"This government is not looking for conflict of that kind."

Analysts said union protests had the power to discomfort thegovernment but not make it change course.

"The days when the unions could hold the government toransom in any serious and long-term way are behind us -- butthat's not to say there couldn't be some incredibly seriousdisruption," said Justin Fisher, a political analyst at BrunelUniversity in southern England.

"It will be unpleasant, I assume, but not threatening to thegovernment." (Additional reporting by Matt Falloon; editing by Paul Taylor)