Forcing Ming Campbell out seems like a very risky strategy to me for the Liberal Democrats. They might get a bit of a boost with a new leader, but whoever they pick will have to overachieve seriously (or be very lucky) to get much of a permanent increase in the opinion polls. The reason they did so well in 2005 were that they weren't Tony Blair or Michael Howard and opposed the war on Iraq.
I don't know how true this is across the country, but in my experience Lib Dem activists had a deep faith in the inexorable advance of their party. Because of the unpopularity of the Tories and Labour's decline in support, in many parts of the country they acquired more and more local councillors and moved from third to second in parliamentary elections. They seemed to think that this process would continue indefinitely, and since 2005 have been finding out the hard way that it doesn't. It might be these inflated expectations that have helped do for Ming.