Mnuchin said Treasury's number one priority will be simplifying the tax code by closing loopholes and creating fewer tax brackets, and creating a middle-class tax cut, while also lowering the corporate tax code to make U.S. companies more competitive around the globe.

Trump to focus on budget priorities in speech

According to media reports, agency heads received total funding limits for the year this week. The White House is expected to release a budget blueprint on March 16, while a full budget with detailed policy prescriptions and economic scoring won't be released until early May.

The White House budget is expected to include a $54 billion increase in defense spending, the cost of which will be offset by reductions in the federal budget's discretionary spending programs, including international aid issued by the State Department and funding for the Environmental Protection Agency, according the numerous reports.

Proponents of fiscal reform have been quick to note the limits to paying for new spending initiatives with cuts the discretionary budget programs, which only account for 15 percent of the total budget. Social Security, Medicare and spending on other government health programs account for 50 percent of the budget.

"It is encouraging the administration is apparently proposing to offset the cost of increased defense spending, indicating they take pay-as-you-go principles seriously, said Maya MacGuineas, president of the Committee for a Responsible Federal Budget, a bi-partisan think thank that advocates for balancing the budget.

But MacGuineas and other budget hawks, including Mick Mulvaney, the head of the Office of Budget and Management, have indicated that a serious attempt to address the country's debt can't be made absent entitlement reform.

"Entitlement reforms are necessary not only to protect those who depend on them but also to slow rising debt levels and create fiscal space for other programs, which are currently being crowded out," said MacGuineas.

401(k) tax-deferred treatment may be on the block

Mnuchin has said an overhaul of the tax code will come by August of this year, but likely won't be enacted until 2018.

The administration's commitment to lower marginal tax rates and simplifying the tax code has many retirement experts fearing that the tax-favored treatment of 401(k) and IRA contributions will be vulnerable under the Trump administration.

At a recent forum hosted by the Employee Benefits Research Institute, Randy Hardock, a partner at Davis & Harman and a former staffer on the Senate Finance Committee, said the tax code's preferences for retirement savings programs will be vulnerable, as Congress and the Trump administration look to offset revenues lost from lower tax rates.

"It is incomprehensible to me that Congress will not go after retirement plans in some way in tax reform," said Hardock at the forum.

Brian Graff, CEO of the American Retirement Association, reiterated the likelihood that gutting tax incentives for retirement savings will be used for pay for overall tax cuts.

Republicans "want to do tax reform that's revenue neutral, they want to lower rates, they want to lower taxes on investments, and they want to lower corporate rates," Graff said at EBRI's event. "To be revenue neutral, that money has to come from someplace else, and historically, retirement tax preferences have been a target to pay for other priorities.

Tax incentives to promote retirement savings are considered to be among the most expensive in terms of foregone federal revenue.

Congress' Joint Committee on Taxation has estimated that tax incentives on qualified retirement contributions cost $170 billion in tax revenue in 2016.

Other projections from the Obama administration's OMB said defined contribution plans will cost $415 billion in tax revenue between 2015 and 2019.

Tax breaks on defined benefit assets will reach $333 billion in that time, and revenues lost on incentives for IRA investors will reach another $98 billion.