The use of computer technology to pinpoint and follow floating voters almost certainly means that candidates breach legal limits on their expenses, say observers. Although each candidate's expenses are strictly limited, national parties can spend what they like.
Andrew Puddephatt, director of the democratic campaign group Charter 88, said the average of pounds 9,000 allowed for each seat was well below what many candidates would spend.
"That's not going to allow you to mailshot voters five or six times, invite them to focus groups, phone them and so on. Parties are going to 'lose' this expenditure by paying for their mailshots and phone banks nationally," he said.
Mr Puddephatt said the parties were now targeting their resources so accurately at undecided voters in marginal seats that only about half a million people - 1 per cent of the population - would bear the brunt of the parties' campaigns.
Both the main parties have already been reported to the police for allegedly spending more than they were allowed at the Wirral South by-election. The UK Independence Party says the Conservatives booked an entire hotel for a week, while Labour canvassers telephoned floating voters up to half a dozen times. The by-election expenses are due to be published on Friday, and the party will scrutinise them to try to identify items which may have been left out.
Chris Rennard, campaigns director for the Liberal Democrats, said his party had spent up to its limit in Wirral but had not been able to target voters as intensively as the other two parties.
They had been able to post mailshots out first class while the Liberal Democrat volunteers were pushing them through doors by hand, he said, and had set up telephone banks from which to phone voters. Labour had had not one campaign office, but several.
"There is a strong case for saying there should be national campaign limits. As it stands, you can have adverts in every newspaper in the country and that isn't charged to candidates' expenses.
"But if you put one advert in a local paper that is chargeable," he said. Mr Rennard added that the Liberal Democrats hoped to raise enough to spend up to their legal limits in key seats, and no more.
Des Wilson, founder of the Campaign for Freedom of Information, also claimed that the parties would overspend. But few people were likely to complain, he said.
"Everyone in politics knows that there is little relationship between the final figures submitted for the costs of electioneering in a marginal seat and what has actually been spent there," he said.
Candidates are allowed to spend around pounds 8,000 on their campaign - pounds 4,965 plus 5.6p per elector in county seats, and 4.2p per elector in boroughs.
Labour said no candidate would breach spending limits.Reuse content