Cameron and Brown both have the same problem in the next two debates. Both have been saying that a LibDem vote is a wasted vote and they have no chance of getting elected, so if they begin to target him it means they take the tyhreat seriously and that their assertion is not true.
Also, the public do not like the political fighting at PMQs and if they round on Clegg, make a victim, it could help him and reflect badly on them.
Then there is the fact that it looks like one of them will need his help in the event of a hung parliament. It they put the boot in now, they may regret it on May 7th.
I suspect he has not peaked too early. They Tories 12 point lead was always going to close the nearer to election day and it has.
Clegg is more of a problem for the Tories, they have been pushing the 'it is time for a change' without spelling out what they intend to do. The Tories have not been radical enough, and are really not much different from Labour, so the voters do not believe the Tories change message. Then along comes Clegg, and says we are the real change, and the voters get it, they are.