* Clegg seeks to end Labour and Conservative dominance
* Liberal Democrats far behind in polls before 2015 election
* Once popular leader weakened by austerity and compromise (Recasts after speech, adds quotes)
By Peter Griffiths
LONDON, Sept 18 (Reuters) - Deputy Prime Minister Nick Clegg, flagging in opinion polls less than two years before an election, vowed on Wednesday to break the hold of Britain's two main parties and keep his Liberal Democrats' place in a coalition government.
Clegg's popularity has slumped and his centrist party has bled support since taking power in 2010 with Prime Minister David Cameron's Conservatives in Britain's first formal coalition since World War Two.
But despite the knocks and bruises he has had to suffer in harness with a larger, centre-right party, Clegg insisted the Liberal Democrats were a party of government.
"We are not here to prop up the two-party system, we are here to bring it down," he told supporters at his party's annual conference in Glasgow, Scotland.
"If we can do this again - in government again in 2015 - we are a step closer to breaking the two-party mould for good."
The Conservatives and Labour have dominated British politics for much of the last century.
In a sign of the trouble that lies ahead for his party, a poll suggested the centrist Liberal Democrats could be wiped out at the 2015 election, finishing in fourth place, behind the small UK Independence Party, an anti-European Union group.
If the next election ends without one party in overall control of parliament, Clegg said he was ready to form a coalition with either Cameron's centre-right Conservatives or Ed Miliband's centre-left Labour.
Clegg emerged from the conference unscathed, winning votes on tax policy and the government's economic plan and brushing aside talk of a challenge by his outspoken Business Secretary Vince Cable.
But the party that took 23 percent of the vote at the 2010 election and secured their first taste of national power for 70 years still has a mountain to climb before the election.
Ranked in the weeks before the last election as the most popular British politician since Winston Churchill, Clegg's appeal has plumetted during his time in government.
His support for unpopular austerity measures, a broken promise not to increase student tuition fees and the daily compromises of coalition government have all taken their toll.
The coalition has also been strained by arguments over the austerity drive, taxes, the European Union and planned reforms to electoral boundaries.
With polls giving Labour only a three or four point lead over the Conservatives, there is a chance that Britain will once again have a coalition government. The Liberal Democrats could play the role of kingmakers, as they did in 2010. (Editing by Ralph Boulton)