Clegg can attack the Tories because he knows they and the LibDems could never form an alliance.
No one seems to notice that Clegg did not attack Labour, he attacked Brown. The Lib Dems and Labour could easily work together, and Brown can be got rid of by the Labour Party by the Conference in the Autumn.
There are 26 LibDems / Tories marginal seats. If the Tories don't get the majority of those, and it is not looking good for them at the moment, then the Tories will not have a big enough swing and they will lose the election. And Cameron will be history before Brown.