The PM says "there will be time later on to talk about these things" when asked if he supports Smith.
One of the few factors that could tilt the odds in Alex Salmond's favour is the prospect of permanent cuts under a Conservative-led government.
The coalition made the Labour leader's point for him as it fielded an entirely male frontbench.
The Labour leader broke with his new sober style and hammered Cameron over his refusal to rule out cutting the top tax rate again.
Of the 24 signatories to the letter attacking Labour's plan to reintroduce the 50p tax rate, eight have donated to the Conservatives.
The Labour leader had no convincing riposte to Cameron's claim that he was "an arsonist" who "complains that the fire brigade aren't putting the fires out fast enough".
Unemployment fell at its fastest rate since 1997 to 7.1% but average earnings rose by just 0.9% - 1.1% below inflation.
Any communications strategy devised in Downing Street also has to compete with noisy agendas elsewhere in the party.
The "truce" between the two leaders lasted just a week as Cameron declared that Miliband had "all the moral authority of the Reverend Flowers".
To date, the coalition has unforgivably weakened Britain's climate adaptation plans.