Demokratika is estimating that the government's proposed tax cuts could cost the country between €100 million (Lm42, 930,000) and €150 million (Lm64,400,000) and would require an economic growth rate of between nine and 13 per cent to "fill the hole".
It invited the Prime Minister to contradict the Green party's estimates or declare how much his proposal would cost.
AD challenged the government to adopt a more serious policy and say how it would finance the tax cuts, calling the announcement a "shock" and a "copycat" proposal.
AD has been proposing tax cuts from the maximum rate of 35 to 30 per cent for small enterprises for the last two years, with the added difference that it was also proposing how to make good for a drop in the government's revenue through a higher tax on the banks.
The government's tax reduction would mean a "significant" decrease in revenue, with no inkling as to how it would compensate for it, AD chairman Harry Vassallo said, describing it as an electoral gimmick he had hoped would have stopped once Malta joined the EU.
Making good for the shortfall in government revenue through growth in the economy was an assumption and not seriousness, AD said.
The trade off would be an increase in the tax compliance rate but factoring in the level of compliance required in the scenario proposed meant an increase from 65 to 86 per cent, according to AD's calculations.
"To think that this could be achieved in a short time span is dreaming," AD's spokesman for finance and economy, Edward Fenech, said.
AD's own proposal had calculated a tax compliance improvement of five per cent in the medium term.
AD expressed its "surprise" at the radical measure on the eve of an election, saying it was reminiscent of the MLP's promise of a two-month tax holiday in the 2003 election - another irresponsible promise.
The Prime Minister's pledge to cut tax by 10 per cent was comparable to the opposition's plan to halve the surcharge. Mr Fenech said the MLP's proposal would actually cost less but AD was militantly against it because it was "incredibly irresponsible".
In the midst of an electoral campaign, things had to be taken with a pinch of salt, Dr Vassallo said. But it was also important that the promises would respect the electorate's intelligence.
AD's proposal did not have an electoral aim but had a fiscal and economic rationale so that Maltese companies would not be disadvantaged by having the highest income tax rates in Europe, compared to 33 per cent in France, 30 per cent in Germany, 12.5 per cent in Ireland and 10 per cent in Cyprus.
AD also questioned whether the government had consulted, or would need to consult, the EU over such a reduction in income tax - a fiscal manoeuvre that could seriously and potentially reduce government income.