The Liberal Democrat leader said the highest earners were still being asked to pay more than during Labour's 13-year reign.
However, he used his keynote speech to his annual conference in Brighton to tell his party grassroots: "There can be no question of lowering [the rate] further during this parliament."
Despite the LibDems' attempts to focus on tax, however, the largest cheer from the party faithful was for Mr Clegg's other major announcement – that former leader Paddy Ashdown will run the party's 2015 General Election campaign.
Party sources denied the role would lead to any kind of Lib-Lab pact talks, of the kind Mr Ashdown held with Tony Blair. They also denied his appointment avoided handing a high-profile platform to any potential leadership challengers, following renewed speculation over Mr Clegg's position.
The LibDem leader has attempted to win back the trust of the British people following last week's much-mocked public apology over tuition fees.
Addressing delegates, he hit out at both Labour and the Tories as he insisted he would take no more lectures on "betrayal".
It was Labour who had betrayed the UK by leaving it penniless, he said.
And he accused the Tories of attempting to fool voters on environmental issues, denouncing what he described as "PR" spin.
However, his strongest message was a nightmarish vision of a future UK, dominated by far-right extremists if it failed to deal with its financial problems. It follows his warnings further cuts after the next election could include cuts to benefits for better-off pensioners.
If the West failed to respond to its economic challenges, he warned, extremists could force a shift "from moderates to hard-liners, from internationalists to isolationists, from those com-mitted to the politics of co-operation to those hell-bent on confrontation."
Aides pointed to the recent rise of far-right nationalist parties in countries such as Greece. Asked about the UK, party sources said: "We have got to guard against that kind of extremism."
After putting his party on a war footing, he also gave a nod to 2015 election strategy. He said delegates should ask voters on the doorsteps: "Are you ready to trust Labour with your money again? And do you really think the Tories will make Britain fairer?"
In a heavily-trailed passage, he also challenged members of his own party to sign up to the difficult decisions that come with government or leave.
Following what has been a tense week for the party, there were a few empty seats for the speech. However, the warm reception for his words prompted even Mr Clegg to describe the hall as a "generous audience".
Labour deputy leader Harriet Harman said voters should not believe "a word the LibDems say". She added: "The truth is they have betrayed the people who voted for them, and far from being a brake on the Tories, they are their accomplices."
Meanwhile, the right-of-centre Institute of Economic Affairs think-tank has called for pensioners to voluntarily surrender universal age-related benefits. The group warned that means-testing the rights, such as to a free bus pass, would be costly and a "bureaucratic nightmare".
l One notable figure who was not at the conference this year was former leader Charles Kennedy. The MP for Ross, Skye and Lochaber, according to his office, was absent in order to deal with "family problems". It is understood Mr Kennedy's parents are not well.
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