OTTAWA - Hoping to whet voters' appetites, the federal Liberals laid out a tantalizing feast of campaign promises in the first two weeks of the election campaign.

But taxpayers should wait before they sit down and tuck in, experts warn. The final bill may be one they simply can't afford.

"They either have to find the savings to pay for their spending promises, or be prepared to raise taxes to fund all their announcements," said Adam Taylor of the Canadian Taxpayers Federation.

The group is still crunching the numbers for all the major parties, but it's difficult to see how the Liberals will be able afford their largesse without pushing the country perilously close to deficit, said Taylor.

Initial estimates suggest the Liberals' campaign promises would cost nearly $15 billion within the first four years of government. But that doesn't includes some big-ticket items like a 10-year, $70-billion plan to invest in infrastructure or boosting the child-care allowance by $350 annually.

Taylor said his group has pegged Liberal spending much higher, at close to $17 billion a year, taking into account all of their pre-election promises, including billions for aboriginal communities under the Kelowna Accord.

The Conservatives, meanwhile, have stuck to a strict diet of tax cuts and modest proposals while on the campaign trail. But they also went on a pre-election spending blitz that the taxpayers federation pegged at a hefty $19 billion.

Still, the Liberals' costly proposals have provided plenty of ammunition for Conservative Leader Stephen Harper, who accused his opponents of making "mind boggling" election promises that would plunge the country into deficit.

Liberal Leader Stephane Dion has defended his platform as "fiscally prudent" and denied he'd run a deficit, pointing out that his party cleaned up the $40-billion deficit problem left by the previous majority Tory government of Brian Mulroney in the early 1990s.

But experts say it's still unclear how the Liberals will pay for their promises without raising taxes or making cuts elsewhere.

Most governments dip into surplus funds to pay for big projects. But federal surpluses are expected to be small, with $2.3 billion projected in this fiscal year and $1.3 billion in 2009-10.

The Conference Board of Canada, a private sector think tank, believes nevertheless that the surplus could come in as high as $5 billion this year, despite the sluggish economy.

"The economy's in substantial turbulence right now," said Arthur Sweetman, a policy studies professor at Queen's University in Kingston, Ont.

"Certainly if things go in a bad way, then almost any promises will put us in a deficit. And I think that if you add up the value of all their promises, the economy needs to be in reasonably good shape for them not to drive us into a deficit."

The Liberals say their Green Shift plan, which promises to offset the cost of a carbon tax with income and business tax cuts, will help finance some of their campaign promises, such as $300 million in energy retrofit tax breaks for homeowners and $400 million for farmers who reduce greenhouse gas emissions.

They're also proposing new ways to fund costly projects, such as selling tax-free "green" bonds which would raise money for environmentally friendly infrastructure projects like renewable energy. But the government also have to borrow $50 billion over five years to help get the proposed infrastructure bank off the ground, Liberals say.

More details will likely emerge once the Liberals releases their full platform Monday. Until then, voters should be wary of politicians promising to keep a tight grip on the country's purse strings, said Sweetman.

"You want a party that's not single-minded, but is flexible and can respond to what seems to be a rapidly changing economic environment."