Nick Clegg has accused the Tories and Labour of being dragged to the "extremes" of right and left in the latest salvo of the battle for votes in Thursday's local elections.
The Deputy Prime Minister laid into his Tory coalition partners over benefits, human rights and the environment, accusing David Cameron of abandoning "compassionate" Conservatism.
And he said Labour under Ed Miliband was becoming a "protest" party, urging Liberal Democrat activists to persuade centre ground voters that only his party now represented them.
"The Liberal Democrats are different. We will not be dragged one way or another," he wrote in an article for the Lib Dem Voice blog, saying the two biggest parties were going "backwards".
"And as the country continues to navigate the most profound economic storm in living memory, we will be the anchor Britain needs: a strong and pragmatic check on both extremes."
In a very direct dig at Mr Cameron, the Deputy Prime Minister said the Tories had shifted away from attempts to "detoxify" the party by "hugging hoodies and frolicking with huskies".
And he seized on the Prime Minister's refusal to consider cutting benefits for wealthy OAPs. "The Tories are pulling to the right in an attempt to appease their base," he wrote.
"Compassionate conservatism has been sidelined. So-called benefits scroungers have been back in the firing line, along with the European Convention on Human Rights. The blue team used to claim to have gone green, yet have now publicly denounced the importance of environmental protections.
"Despite millions of ordinary families feeling the pinch, the Conservatives resist making the tax and welfare systems fairer still - ruling out introducing a Mansion Tax or looking again at the benefits paid to very wealthy, even multi-millionaire, pensioners."
Highlighting criticism of Mr Miliband by former Labour prime minister Tony Blair, he said: "By offering anger rather than hope, Labour are steadily becoming a party of protest. They are making the classic mistake of opposition, talking only to themselves rather than setting out a positive vision for the nation. Their absence of ideas only confirms that they cannot be trusted on the biggest challenge of our time: fixing the mess in the economy they helped create."