Next, they need to attack specific Lib Dem policies; not in terms of details but values. For example, the Lib Dems plan to avoid sending criminals to prison for crimes that currently carry less than a six month sentence. This means that someone could assault a police officer and not go to jail. That's hardly the position of a party that is on the side of the law-abiding majority.
Finally, Cameron should show those toying with voting for the Lib Dems that he'll deliver many of the things they want. That he'll clean up politics, restore the liberties that have been lost these past 13 years, keep Britain at the forefront of the fight against global poverty unlike the Lib Dems who'll cut the DFID budget, make Britain a greener, healthier place to live and hand power from the centre to local communities.
At the moment the Tory strategy revolves around warning of a hung Parliament. There's no doubt that a hung Parliament would cause massive problems and risk market panic. But the electorate right now is not keen to feel like it is being bullied by the City and the markets, things they blame-rightly or wrongly-for the current economic problems. Also given Clegg's personal popularity, the idea of Labour or the Tories having to listen to him, include him in government is hardly unappealing to voters.
The rise in Lib Dem support rather reminds me of tulip mania. This burst in popularity has made Clegg the most popular party leader since Churchill. It is hard to imagine that there won't be a market correction at some point. But the Tories need to catalyse this.