Andrew Grice: The ghosts of politics past that haunt Labour and Tory election campaigns

Sometimes the small things tell us big things. The script for Labour politicians is to frame "the choice" at the general election as between "Labour and the Conservatives", while the Tories invite people to choose between "another five years of Gordon Brown" and "David Cameron and change".

Labour is confident that voters like its values if not its leader, who is the party's weak point. Conversely, the Tories have bet the farm on Mr Cameron, their prized asset and symbol of change – or so they hope. Their Achilles' heel, and the Labour attack they most fear, is "same old Tories". It undermines their central pitch that, because Mr Cameron has changed his party, both he and it are fit to change the country.

Both main parties are being haunted by returning ghosts. Labour's election preparations risk being derailed by the strikes at British Airways and on the railways, allowing the Tories to revel in a "spring of discontent". Meanwhile, the row over Lord Ashcroft's tax status, skilfully resurrected by Alistair Darling in his Budget, casts doubt over the Tories' claim no longer to be the party of the rich. Focus groups tell Labour that people thought Mr Cameron had "sorted" the problem over his party's deputy chairman years ago. The "Tory toff" card, if played subtly rather than shrieked desperately by Labour, may damage Mr Cameron. Some voters see him as a man who looks after his friends.

How dangerous are the old ghosts? Tory warnings about a "return to the 1970s", gleefully taken up by right-wing newspapers, may strike a chord but will mean less to a first-time voter born in 1992. True, some messages are handed down the generations. Voting Tory in Scotland is unthinkable for many, even though they can't remember a Tory government. Folk memories of an uncaring Thatcher government play on the minds of some people who can't recall it as they weigh up Mr Cameron.

Mr Darling's admission that Labour's cuts will be "tougher and deeper" than Baroness Thatcher's was not exactly in Labour's election handbook. This now includes what he should have said: cuts in some budgets will be as deep as Lady Thatcher's in order to protect vital frontline services. The unions are a headache for Labour but if Mr Brown and his ministers continue to distance themselves from the strikes, they may be less damaging to the party's prospects than the Tories hope. Yes, Labour is heavily dependent on the unions for funding. It had nowhere else to turn when big individual donations dried up after the "cash for honours" affair engulfed Tony Blair's government. But the number of strikes is tiny compared with the 1970s and nobody can say the unions have called the shots since 1997.

For the most part, they have been the dog that didn't bark – until now. Plenty of the unions' policy demands won little sympathy from the Government, which is why Bob Crow, now leading the rail workers into industrial action after Easter, took the RMT's ball away and disaffiliated from Labour in 2004.

What cabinet ministers fear most is that the strikes will drown out Labour's campaign messages, such as the pledges that Mr Brown will speak about today. The Tories had a similar problem with the Ashcroft row. A party leader might want to talk about his shiny new policy, but the media may decide that strikes or tax havens are more sexy.

Labour had another headache this week when a ghost which stalked the last Tory government under John Major – sleaze – turned up unexpectedly on its doorstep. The "Lobbygate" exposé of the former cabinet ministers Stephen Byers, Patricia Hewitt and Geoff Hoon apparently touting for business looked very damaging. It seemed like the final chapter of a story which began when Mr Blair said New Labour had to be "whiter than white" and the people's servants not their masters after evicting the sleaze-ridden Tories. Animal Farm, perhaps.

Today, the voters think all politicians have their snouts in the trough. Labour's polling suggests that last year's scandal over MPs' expenses hurt the party not so much because it is the governing party or has more MPs, but because people associated sleaze with the Tories and thought Labour was different. Now they think all politicians are as bad as one another. "A plague on both your houses", was a phrase used in both the Brown and Cameron inner circles this week as they assessed the fall-out from Lobbygate. Reports from the election front line suggest that MPs' expenses is still a big issue for many voters, a stinky symbol of their distaste for the political class as a whole – and surely a major reason why the Tories are not streets ahead in the opinion polls.

The strikes, the lobbying scandal and a neutral Budget that was never going to be a game-changer do not appear to have harmed Labour's standing. If anything, the Tories' poll lead has narrowed this week and a hung parliament looks increasingly likely. Why?

The only answer is that, for all the negative feelings towards Mr Brown, there is little enthusiasm for the Tories. Mr Cameron has 40 days to persuade people he offers real change and to generate some positives by answering the "change to what?" question.