It will terrify Cameron that even after making so many concessions to the right, the Tories were still beaten by UKIP.
The Defence Secretary's intervention puts pressure on Clegg's party to keep its pledge to prevent further welfare cuts in this summer's Spending Review.
If Cameron wants to show global leadership on aid, he needs to start by showing leadership in his own Parliament and seeing off the Tory opposition.
A new poll from Lord Ashcroft shows seven per cent of Lib Dem voters and 10 per cent of UKIP voters expect to vote Conservative at the general election.
When Conservatives, including Cameron, indicate their eagerness to be a little bit more like UKIP, all they do is give Nigel Farage the credit for setting the agenda.
If the Lib Dems benefit from an incumbency factor and UKIP splits the right-wing vote, the Tories will be the big losers.
The PM faces a backlash after the Lib Dems win the by-election and the Tories finish behind UKIP in third place.
A new poll from Lord Ashcroft putting the Lib Dems five points ahead in the by-election shows that local issues continue to take precedence over national ones.
The Huhne and Rennard scandals mean defeat will be portrayed as a humiliating set-back for Cameron.