* Liberal Democrats bidding for second stint in coalition
* Party popularity has slumped since 2010 election
* Symptomatic of slide, Clegg's own seat under threat
* New campaign, local approach may limit electoral damage
By William James
SHEFFIELD, England, Feb 5 (Reuters) - Britain's Deputy Prime Minister has two battles to win by May. Not only must he win back voters who feel betrayed by his support for big public spending cuts, but he also has to hang on to his own parliamentary seat.
Nick Clegg's centrist Liberal Democrats will have been in coalition government with the centre-right, anti-welfare state Conservatives for five years when the next general election comes around. For many of his former supporters, another five years is out of the question.
"I've voted Lib Dem before, but I'm so fed up with Mr Clegg I don't know what I'm going to do this time ... Sometimes I could slap him!" retiree Elizabeth Craghill told canvassers braving a blizzard to knock on doors in Clegg's constituency of Sheffield in north England.
The fight is a hard one. But with neither the Conservatives nor Labour forecast to win the election outright, Clegg still hopes he can win five more years in coalition at the helm of the world's sixth-largest economy by calibrating his manifesto to let him cut a deal with either party.
If he is successful, the Liberal Democrats could nudge Britain towards the centre ground with their creed of staying in the European Union and promoting fiscal policy that blends tax rises with spending cuts to eliminate the budget deficit.
The challenges Clegg faces to even get to the negotiating table are crystallised 160 miles (260 km) north of London in Sheffield, a once-mighty industrial city that has struggled to adapt to the decline of its world-famous steel mills.
"People here are incredibly frustrated that they voted for a man who then turned his back on the promises he made," says Oliver Coppard, the 33-year-old local man who is standing for Labour candidate and hoping to oust Clegg, a Cambridge-educated, former European bureaucrat.
Coppard has gained national attention for his campaign since a surprise poll in November showed Clegg's lead had shrunk to 3 percent. A poll published on Thursday by Survation on behalf of Labour's trade union backers Unite, now says the Liberal Democrats are 10 percent behind Labour in the constituency.
Nationally, the Liberal Democrats' share of the vote has slumped to less than 10 percent, down from the 24 percent they won at the 2010 election. Polls show Clegg has gone from the most popular party leader in 2010 to the least liked in 2015.
Clegg's declining personal rating reflects the struggle to adapt the party to its new role as part of the establishment.
At past elections Lib Dems had three main sources of support: wavering voters from centre-left Labour and centre-right Conservatives, and protest votes from those fed up with a two-party system that has dominated British politics for years.
Since then however protest voters have been swayed to parties like the right-wing UK Independence Party (UKIP) and the left-wing Green Party. Others, objecting to the LibDem's choice to enter a Conservative-led coalition, have turned to Labour.
Chris Bowers, a Liberal Democrat candidate in the south coast town of Brighton told Reuters the campaigning strategy in his area was not to include Clegg's face on any campaign material and focus on local issues.
That approach is a far cry from 2010, when the media coined the term "Cleggmania" to describe 48 year-old Clegg's popularity after successful televised debates against Labour's then-Prime Minister Gordon Brown and Conservative leader David Cameron, who subsequently became Prime Minister.
"Never heard of it," says 21 year-old Sheffield student Sophie Robinson when asked about the term. "I watch the news quite a lot and I never see Nick Clegg on the telly, I just see David Cameron and I forget it's a coalition."
She wants Labour to win the election.
Activists can reel off a list of the Liberal Democrats' achievements in government: increased income tax relief, funding for disadvantaged school children and childcare reforms.
Voters, on the other hand, can often only recall one: a broken promise to oppose a rise in university tuition fees.
"He got a lot of the student vote secured on the basis of his policy about student fees, and then once he got into power he just said it wouldn't be possible," said Lucas Busetto, a 22-year old student studying in Sheffield.
He said the Lib Dems would be "completely off the radar" for him at May's vote, adding that he was considering voting for Labour or an anti-austerity party.
Should Clegg lose his seat, his likely successors range across the ideological spectrum, potentially making coalition negotiations more difficult. One potential future leader, Danny Alexander, has been criticised for "going native" on the right of the party alongside his Conservative colleagues. Another, Tim Farron, represents a sizeable left-leaning contingent.
In Clegg's Sheffield Hallam constituency, where the Labour-controlled city housing estates turn into a prosperous suburb in the rolling countryside of a national park, the party is defending a large majority of 15,000 votes.
"In 2010 we had a much lighter campaign by this stage. We were so far ahead we didn't need to do much," says Andrew Sangar, a Liberal Democrat councillor responsible for Clegg's local campaign. "This is more about working it hard, not taking it easy."
One voter, solicitor Tim Gaubert, told canvassers that he was switching to Labour because he was disappointed the Liberal Democrats had not done more to oppose criminal and justice system reforms pushed through parliament by the Conservatives.
"It's much easier for us to defend party policy, but when it's coalition policy we have to accept it is what it is and that some people find it difficult," councillor Sangar said, walking away from Gaubert's house.
Nonetheless, party activists are optimistic they can remain a party of government, holding on to enough seats to become a viable coalition partner.
A senior Liberal Democrat source said Clegg had put the "cleverest man in the party", politician David Laws, in charge of making sure the 2015 manifesto could be delivered in partnership with either Labour or the Conservatives.
"He's literally going through every line: 'Is it credible? Is it deliverable?'" the source said.
The party is also concentrating campaign resources into building local strongholds rather than winning thinly-spread national approval, hoping to upset the polls that put the party in fourth or fifth place nationally.
Activists say their efforts dealing with important local issues like street lighting and road repairs will help them get over the line in Sheffield and other party seats across the country.
"That's the only reason I vote for the Lib Dems - because I think they do a brilliant job locally," conceded Elizabeth Craghill to canvassers in the end.
Then she sighed and said: "I'm still not sure about Mr Clegg. But we'll probably end up voting for him...just to keep the others out." (Editing by Guy Faulconbridge and Sophie Walker)