For weeks, speculation had been swirling that the Liberals wanted to get rid of the tax exemption for private health and dental plans, which the Finance Department has calculated costs the federal government about $2.9 billion in foregone revenue per year.

The controversy over the suggestion, first raised by the National Post in December, took on steam as the Conservatives pressed the Liberals to divulge their plans and the government refused to clear things up one way or the other – until they did.

“We are committed to protecting the middle class from increased taxes and that is why we will not be raising (those) taxes,” Prime Minister Justin Trudeau said Feb. 1 in the House of Commons.

The thing is, that while taxing health and dental benefits now appears to be definitively off the table, no one really knows for sure what else is on it as the Liberals work on fulfilling a campaign pledge to find $3 billion per year in new revenue by fiscal 2019-20 through “an overdue and wide-ranging review” of existing tax expenditures.

That is a form of government spending that is done through the tax code, such as deductions or credits, and it is costing the federal government more than $100 billion per year.

The campaign platform said the goal of the review would be to find ways to reduce tax benefits for the rich – defined as individuals who earn more than $200,000 in annual income.

Their first federal budget announced the elimination of tax credits for education, textbooks, arts and fitness – and made it clear that was just the beginning.

Last June, Finance Minister Bill Morneau launched a comprehensive review of the tax system and convened seven experts to give him advice on where things should go.

“This review is part of a broader government commitment to eliminate poorly targeted and inefficient programs, wasteful spending, and ineffective and obsolete government initiatives,” said the news release.

The secret nature of that review has fuelled the political fires on the opposition benches, especially since its results – and how the Liberal government responds to them – could end up being a major component to the upcoming federal budget.

Dan Albas, the Conservative deputy finance critic, said the Liberals might end up going overboard as a way to increase revenue without making the more noticeable move of raising personal income taxes in order to reduce a bigger-than-promised deficit.

Alexandre Boulerice, the NDP finance critic, said his party wants to see the Liberals force senior executives in the private sector to pay taxes on all of the money they earn from stock options, and at the same time increase the working income tax benefit.

The Liberals had promised to fully tax annual stock option gains over $100,000, but then backed down on the pledge in last year’s budget under pressure from startups who feared for their ability to grow.

That kind of pressure could play into this budget too.

Gabe Hayos, vice-president of taxation at Chartered Professional Accountants Canada, said the fact that President Donald Trump is promising a lower-tax environment in the U.S. means the Liberal government would want to think carefully about what they do with tax expenditures that could have an impact on competitiveness.

“Canada is going to need to be very flexible about what it does,” Hayos said.

Conservative leadership candidate Maxime Bernier, meanwhile, said he would review dozens of tax credits worth some $20 billion to help cover the cost of cutting income taxes in general.

Annie Donolo, a spokeswoman for Morneau, said she could not provide any details of the review but noted the government is “not looking at any measures in isolation and no final decisions have been made.”

There are nonetheless some tea leaves available to read.

The Liberal platform mentions a 2014 analysis of tax expenditures from the parliamentary budget officer, which suggests that pension income splitting and the dividend tax credit in particular provide much greater benefits to those with larger incomes.

Kevin Milligan, an economist from the University of British Columbia who was named to the review panel at Finance Canada, wrote a paper for the C.D. Howe Institute in 2014 that makes the case for eliminating boutique tax credits like those for public transit and volunteer firefighters, while acknowledging the political cost of doing so.

Milligan also argued that since the age amount and the pension income amount are non-refundable, they cut out many seniors who could use the additional financial support. He suggested it would be better to move their value into the Guaranteed Income Supplement and Old Age Security.

Jennifer Robson, an assistant professor of political management at Carleton University who is also on the review panel, wrote a piece for Canada 2020 in 2015 that highlights the public transit tax credit and the Canada employment credit as particularly regressive.

Robson, who made it clear she was not speaking specifically about the work of the review panel, said it is about more than setting and meeting a savings target.

“I think that there is value in thinking of this work of looking at tax expenditures and credits from the perspective of simplicity and reducing burdens of compliance on individual taxpayers as well as businesses,” she said.

The Macdonald-Laurier Institute published a paper last week that sets out a checklist for deciding which ones should stay, change, or go.

Its author, Sean Speer, concluded the government should keep the charitable donations tax credit and the exemption from paying capital gains tax on a primary residence.

He said the government should make the disability tax credit refundable and also start taxing health and dental benefits provided by employers, while using that extra revenue to give a refundable tax credit to those who have to buy insurance on their own.

Meanwhile, Speer argued, the age credit and the public transit tax credit should be eliminated.

Like Albas, Speer pointed out the fiscal situation has changed quite a bit since the Liberal campaign promise, but he came to the opposite conclusion about how the government might react.

“If it’s not going to push you into a surplus leading up to an election, I’m not sure it’s the type of thing you would want to spend a lot of political capital on,” said Speer, who was economic adviser to former prime minister Stephen Harper.