Today's campaigning isn't about which tax policy you support. It's about who you believe.
Every general election campaign inevitably features rhetoric about the "deception" of "misleading" opposing parties. Usually this builds to a crescendo of bile as polling day approaches. It's an indication of quite how tight this race is that, this time round, the shouting match is already at full volume.
The Conservatives are attracting the most flak, with both Labour and the Liberal Democrats turning their guns on Tory spending plans. Labour's message from Gordon Brown, Alistair Darling and Peter Mandelson was that these have the strength of a "house of cards".
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Labour's argument, in short, is that with £15 billion of efficiency savings already being introduced in the 2010/11 financial year there's simply no room to find another £12 billion without putting the recovery at risk.
At the heart of their argument was the claim that the Conservatives were being dishonest.
The prime minister said the Conservatives are "misleading people about the future". The chancellor said if he had announced this kind of "uncosted giveaway" in the Budget he would have been laughed out of parliament. And the first secretary of state Lord Mandelson went furthest of all, expressing open disdain at "the deception they're pedalling".
David Cameron responded to the claims later this morning by effectively making light of the issue. These cuts are only £1 in every £100 - can't the government copy what businesses and households have been doing and tighten the belt a little? "To save £6 billion a year is not a challenging target," he said dismissively.
Is it, or isn't it? Can it be done, or can't it? This is not a debate about matters of principle, but about what is and what isn't possible. It's about who you believe.
The Liberal Democrats offered a variation on the theme this morning. They are huffing and puffing by pointing out that, unlike their fiscal plans, Tory proposals are not tax-neutral. These will cost £13.5 billion, but - according to the Lib Dems - only £100 million of alternative new tax revenue have been identified by the Conservatives.
"Their tax promises on marriage and jobs may sound appealing. But they come with a secret VAT bombshell close behind," Nick Clegg said alarmingly.
A VAT bombshell? I don't remember George Osborne announcing anything like that. The Lib Dems have simply calculated how much £13.4 billion would cost if the bill was slapped on to VAT (which would have to go up by three per cent). They might be assuming a bit too much, but the point is the money would have to come from somewhere; each household would pay an extra £389 every year.
Is that really plausible? How are the Conservatives going to pay for their tax plans? It's about what has been announced and what hasn't. It's about who you believe.
The Conservatives, as you'd expect, are continuing to make hay on what they call Labour's "jobs tax" - the proposed hike in national insurance (NI) contributions which has so upset business leaders. Another 13 titans of industry added their signatures to the Telegraph letter today, following the 30 additions bandied about by Cameron in PMQs yesterday.
"The growing weight of business opinion is that it's right to cut waste and stop the taxes, rather than go on wasting and put up the taxes. That argument is an argument the government is roundly losing," Cameron said.
Here is the exception which proves the rule. The reason the Conservatives are doing so well out of the NI debate is precisely because they're attracting support from outside politics.
The chancellor pointed out this morning that businesses are only one sector of British society; that it's "inevitable" they will disagree "some of the time". The point is, they are part of a sector which is especially good at making its opinion heard.
The Tories benefit from the fact that, on NI, it's not about which of the parties you believe - because of the intervention from business leaders which gives their argument extra credence. As Mandelson said bizarrely, "it's not about politics". Businesses are removed from the partisan struggles which discoloured Labour and the Lib Dems' offensives today.
It's those signatures from figures outside Westminster which are helping the public choose who to believe, not the to-ing and fro-ing of everyday political campaigning. That's good news for the Tories - and helps raise them above the fray entered into so enthusiastically by their attackers.