Nick Clegg is facing questions over his funding arrangements, after it emerged that a series of payments from party donors were made directly into his private bank account.
The revelation, in the Daily Telegraph, was one of a rash of front pages putting the Lib Dem leader under intense scrutiny as he prepared for the second live TV debate of the General Election campaign.
The Daily Express branded Lib Dem immigration policies "crazy" and The Sun accused him of "wobbling", while the Daily Mail revived comments he made about Anglo-German relations in 2002 under the headline "Nick Clegg in Nazi slur on Britain".
Liberal Democrats claimed their leader was being "smeared" in response to his success in threatening the dominance of the two biggest parties at Westminster.
A Lib Dem spokesman said: "We believe that a concerted effort at political smear is under way, but we are confident the public will see it for what it is - a kickback by vested interests against the prospect of change."
The Daily Telegraph revealed that three Lib Dem donors paid up to £250 a month into Mr Clegg's account before he became party leader.
Copies of Mr Clegg's personal bank statements submitted during 2006 to back up his expenses claims recorded "automated payments" from Ian Wright, a senior executive at drinks firm Diageo; Neil Sherlock, the head of public affairs at accountants KPMG; and Michael Young, a former gold-mining executive.
All three men confirmed that they had paid the money to help fund a member of staff for Mr Clegg's parliamentary office when he was Lib Dem home affairs spokesman.
A Liberal Democrat spokesman said: "The donations were properly made and declared and were used to fund part of the salary of an additional member of Nick Clegg's parliamentary staff.
"The Telegraph story is wrong in fact and we regard any implication of impropriety as unacceptable."