The latest BBC poll of polls has Labour on 34%, the Conservatives on 33%, UKIP on 14%, the Lib Dems on 8% and the Greens on 5%
UKIP leader Nigel Farage told the BBC his party would scrap inheritance tax and says details of how it would be financed would be unveiled in its manifesto on Thursday
Liberal Democrat Chief Secretary to the Treasury Danny Alexander said the Conservatives had "the wrong priorities".
Meanwhile, Paul Johnson, director of the the Institute for Fiscal Studies (IFS) said the debate from all parties around tax in the election campaign had been "deeply depressing".
He told the BBC: "You've had the main parties ruling out whole hosts of relatively straightforward ways of increasing tax, talking about raising tax from some other group, be they the rich or the non-doms or the tax avoiders, but this is all real money and has real effects on the economy - and no sense from anybody about a serious way forward for the tax system."
Of the inheritance tax policy, an IFS note said: "This (and in fact any) IHT cut will also go disproportionately to those towards the top of the income distribution", while Mr Johnson added: "Anything... which increases the tax privilege associated with an asset like housing will drive the price up in the long run."
The £1bn cost of the Conservative policy will be paid for by reducing tax relief on the pension contributions of people earning more than £150,000, says the party, which will make the the policy a key plank of its manifesto.
At present, inheritance tax is payable at 40% on the value of an estate in excess of the tax-free allowance of £325,000 per person. Married couples and civil partners can pass the allowance on to each other.
If the Conservatives win the general election, then from April 2017 parents would each be offered a further £175,000 "family home allowance" to enable them to pass property on to children tax-free after their death.
This could be added to the existing £325,000 inheritance tax threshold, bringing the total transferable tax-free allowance from both parents in a married couple or civil partnership to £1m.
The full amount would be transferable even if one spouse had died before the policy came into effect, the Conservatives say, and so would benefit existing widows and widowers.
For properties worth more than £2m, the new allowance would be gradually reduced so that those with homes worth more than £2.35m would not benefit at all.
Analysis, by Carole Walker, Conservative campaign correspondent
David Cameron will be hoping his pledge to take the family home out of inheritance tax will galvanise his campaign in a week which will be critical for his prospects of staying in power.
When George Osborne promised to raise the inheritance tax threshold to £1m back in 2007, he delivered a huge boost to Tory morale and put Gordon Brown off holding a snap election. Subsequent promises to take more people out of the tax have not been delivered. The threshold has been frozen since 2009 and rising property prices have dragged tens of thousands more families into the inheritance tax net.
His opponents are already questioning whether voters will believe this latest promise. The announcement is likely to appeal to middle-class voters, particularly in London and the South East. But it also opens the Conservatives to the charge they are helping the wealthy. David Cameron's team deny this - pointing out that it will be paid for by reducing pension tax relief for high earners and that many ordinary families now have to pay the tax.
It is, however, a marked contrast to the promises from Labour and the Liberal Democrats to impose a "mansion tax" on expensive properties.
Mr Osborne told BBC One's Andrew Marr Show: "Conservatives support the basic human instinct to provide for your children. And we believe that your home that you've worked for and you've saved for should belong to you and your family, not the tax man.
"So we will take family homes out of inheritance tax, we will effectively increase the inheritance tax threshold to £1m, so that only millionaires pay inheritance tax."
It was a message repeated in a speech by Mr Cameron, who said: "You should be able to pass it (your home) onto your children. And with the Conservatives, the tax man will not get his hands on it."
But shadow treasury secretary Chris Leslie, for Labour, said the policy was the latest "panicky promise" from the Conservatives.
He said: "The Tories made a promise on inheritance tax before the last election and they broke it.
"At a time when our NHS is in crisis and most working people are paying more under the Tories, it cannot be a priority to spend £1bn on a policy which the Treasury says would not apply to 90% of estates."
Danny Alexander said: "It is extraordinary that the Tories will go into great detail on a policy that will cut tax for a small number of estates, but steadfastly refuse to give any detail at all on the massive cuts to public services that they desire."
However, when asked whether the Lib Dems would block the proposals if they ended up back in coalition, he declined to say he would, instead saying: "I'm saying I strongly disagree with it. Our priority... is further increases in the income tax personal allowance... we've stopped things in this parliament including cuts to inheritance tax for millionaires."