It’s all about trust. Many people don't like Mr Cameron [the Tory leader] and they are not sure whether he is genuine — focus groups always mention the car that followed him with his shoes and briefcase as he cycled to work. Others are attracted by the Tory leader, but are sceptical about whether his party has lost its “nasty” instincts. In a recent poll, only 26 per cent of voters said they thought George Osborne was honest, despite his deliberately candid commitment to cuts in his party conference speech. Labour’s private polling suggests that more than a third of swing voters think the Conservatives are not being straight about what they would do in power.

Mr Cameron’s opaqueness over the Lisbon treaty — when it was abundantly clear that there would be no point holding a referendum once the thing had been ratified — mattered because it made him look slippery. When it turned out that the cast-iron commitment had rusted, it just confirmed the public’s view that “politicians are all the same”.

The Conservatives are sticking like glue to their promise to abolish inheritance tax for all but millionaires, despite the financial cost, because they believe they can’t afford politically to abandon another high-profile pledge. They are in a similar bind over the commitment to recognise marriage in the tax system.

Party strategists know that nothing matters more than credibility. Authenticity is the political holy grail. The MPs’ expenses scandal has tarnished all parties, but it has also made it harder for Mr Cameron to present himself as a “fresh start”; a “straight kinda guy”. “This whole awful year has been about trust, whether it’s about the bankers, TV programmes or MPs,” says a Shadow Cabinet minister. “We have to start trusting people more, but we also have to give them reason to trust us.”

This is why the Tory leader’s behaviour over Lord Ashcroft is so bizarre. At the weekend Mr Cameron said that he would, if elected, introduce a law that would require MPs and peers to pay tax in this country. Indeed, the Conservatives agreed yesterday to table an amendment to the Constitutional Renewal Bill in an attempt to force a change on to the statute book before the election. They could do so as early as this week. Lord Strathclyde, the leader of the Tory peers, told me that he expected his party’s vice-chairman to vote for the reform. “There’s no reason why Lord Ashcroft shouldn’t vote for it. He has never voiced any opposition to any of this,” he said. “Tory MPs and peers will support the change. My view has always been that UK legislators should pay UK tax.”

And yet Lord Ashcroft, a man who has given more than £10 million to the Conservative Party, who will be helping to fund the Tories’ election campaign and who is masterminding the crucial operation in marginal seats, still refuses to say whether he currently lives in this country for tax purposes. On his website he states that “if home is where the heart is then Belize is my home”, which rather suggests that his allegiance lies elsewhere. Yesterday, his spokesman repeated the line that his tax status was a “private matter” between the peer and the taxman, but in fact it is a public issue because it was a condition of his elevation to the House of Lords nine years ago that he would “take up permanent residence” in the UK.

This is not just about one man and his tax status; it’s about leadership. The Tory message is that politics must be more open and transparent. Mr Cameron was the first leader to tell his MPs to publish their expenses claims. Mr Osborne has promised to put details of public spending online. “Sunlight is the best disinfectant,” party strategists like to say. Zac Goldsmith was forced to rearrange his financial affairs sharpish when it emerged that he was a non-dom.

Mr Cameron clearly thinks the tax status of parliamentarians matters, or he wouldn’t want to change the law. And yet he cannot or will not say whether a peer who is one of his party’s biggest donors has a financial stake in the country he wants to run. Whenever Shadow ministers are quizzed about Lord Ashcroft they are unable to give a straight answer to a straight question, and therefore sound evasive. “I can only conclude that Michael Ashcroft is the only Conservative in the country that David Cameron is frightened of,” says one frontbencher.

Of course, most voters have never heard of Lord Ashcroft — although you can be sure that by the time the country goes to the polls Labour and the Liberal Democrats will have made him a household name. But the questions about his tax status play into the wider issue of trust. Voters are still unsure whether Mr Cameron really calls the shots with his rightwingers. If he can’t even force his own vice-chairman to say where he lives for tax purposes, then it is fair to wonder whether he could assert himself on things that really mattered if he got to No 10.

-- with some input from agencies