* Liberal Democrat leader faces newspaper backlash
* Clegg tells voters not to be "bullied"
* Labour's Mandelson says attacks "disgusting"
(Adds Mandelson comments)
By Keith Weir
LONDON, April 22 (Reuters) - Liberal Democrat leader Nick Clegg experienced the downside of his new popularity on Thursday when Britain's Conservative-supporting newspapers savaged him.
Clegg, 43, has been projected from outsider to central player ahead of Britain's May 6 election after an assured performance in last week's televised leaders' debate.
Britain's electoral system means the Liberal Democrats are almost certain not to win the election but they could end up holding the balance of power if the vote proves inconclusive.
Clegg woke up on Thursday to find himself the target of a number of negative newspaper headlines. Analysts said the Liberal Democrats must live with greater media scrutiny.
His party issued a statement denying any wrongdoing over a Daily Telegraph report that party donors had paid money directly into his bank account.
The Daily Express headlined "Clegg's Crazy Immigration Policy" and tabloid rival the Daily Mail led with "Clegg in Nazi Slur on Britain", referring to an article he wrote in 2002 when a member of the European Parliament.
Clegg wrote that Britain had "a misplaced sense of superiority" stemming from the defeat of Germany in 1945.
Clegg, 43, was philosophical about the backlash after newspapers had a few days ago compared his popularity with that of wartime leader Winston Churchill.
"I must be the first politician who's gone from being Churchill to being a Nazi in under a week," Clegg told reporters. "I hope people won't be bullied and be frightened into not choosing something different."
Senior Labour politician Peter Mandelson also came to Clegg's defence, prompting fresh talk of a post-election pact between the two parties.
"I think the coverage is frankly disgusting," he told BBC radio. "The press stories we've seen today are straight out of the Tory (Conservative) party dirty tricks manual. These things do not happen at the drop of a hat."
Britain's newspapers traditionally nail their political colours clearly to their mastheads. The vast majority traditionally support the Conservatives with a couple backing the ruling Labour party.
"One of the Tories' great assets is having the vast majority of the press supporting them and they haven't really exploited it until now," said Ivor Gaber, professor of political campaigning at London's City University.
The top-selling daily The Sun, part of Rupert Murdoch's News Corp (NWSA.O), said last September that it was backing David Cameron's opposition Conservatives.
Media commentators said the assault on Clegg indicated that the Liberal Democrat advance had unsettled executives who had anticipated a Conservative win to end 13 years of Labour rule.
"It's to be expected but there is just a hint of desperation among the Tory-leaning press that has spread to the broadsheets," said Steven Barnett, professor of communications at the University of Westminster.
The Sun famously undermined Labour leader Neil Kinnock before the 1992 election and then trumpeted that it had won the election for the Conservatives when Labour slumped to defeat.
Barnett noted that newspaper circulation had declined since the early 1990s and that their influence might be on the wane.
"This grand announcement that they (The Sun) were going to switch their support has not had the kind of purchase on public opinion that they thought," he added.
Nicholas Jones, a former BBC political journalist, said it was wrong for the Liberal Democrats to talk of smears.
"It's up to the Lib Dems to be able to withstand this and to counter this," Jones told Reuters. "That's the price you have to pay in big-time politics in this country where we have a tradition of campaigning journalism." (Editing by Charles Dick)