Why have Mandelson and Clarke been kept apart?

Reading Bagehot a couple of weeks ago, on the prospects for a televisual clash between Ken Clarke and Peter Mandelson, it struck me how the dynamics of the situation have changed. 

Despite the drooling anticipation of numerous political observers, both Labour and the Tories have seemed reluctant to put their two business frontbenchers up against each other.  But, at first, most of that reluctance came from Tory folk I chatted to – their fear being that Mandelson, face-to-face with Ken, would milk the occasion for all its split-story worth.

Now, though, after weeks of strong message discipline from Clarke, that fear seems to have subsided.  Instead, there’s much more reluctance on the part of Labour people I know.  Their worry is simply that Clarke would outperform Mandelson in live debate.  And – given how prominent the two politicians are – that could be damaging for the Government’s economic credibility more generally.