WASHINGTON – The Trump administration's early budget plans for 2018 are already catching heat on Capitol Hill, from Democrats for domestic cuts and from the Republicans in charge of the armed services committees, who say it shortchanges the military.

White House officials say they plan to increase the base defense budget by $54 billion to $603 billion by slashing domestic spending and foreign aid. That netted a swift rebuke from House Armed Services Committee Chairman Mac Thornberry, R-Texas, and his Senate counterpart, Chairman Sen. John McCain, R-Ariz., who have each proposed a $640 billion base budget and argue this is a meager $18 billion more than President Obama had planned.

McCain, in a statement, said the figure was, "a mere 3 percent above President Obama's defense budget, which has left our military underfunded, undersized, and unready to confront threats to our national security... With a world on fire, America cannot secure peace through strength with just 3 percent more than President Obama's budget. We can and must do better."

The comments came as White House budget director Mick Mulvaney told reporters Monday the budget blueprint represents President Trump's priorities, including the military, the nuclear arsenal and border security  –  without adding to the 2018 deficit. The non-defense side of the budget will be $462 billion, Mulvaney said. 

"We are taking his words and turning them into policies and dollars," he said. "It reduces money to other nations, eliminates duplicative programs and eliminates programs that just don't work."

Asked how the defense top-line would carry forward Trump's pledge to rebuild the military, Mulvaney said it was too soon to say. The Defense Department would over the next two weeks craft its budget to the top-line provided by the White House. The White House will issue a fuller budget proposal in early May.

Big questions about the plan remain, including whether that defense money would be just for the Pentagon or for broader defense programs such as the Department of Energy's National Nuclear Security Administration; whether foreign aid and security assistance programs from the Department of State could be shifted over to DoD; or whether the money will flow into the Pentagon's base budget or the Overseas Contingency Operations fund.

The biggest question of all — how a proposal that guts the non-defense budget would survive on Capitol Hill — suggests the White House has offered an opening negotiation position rather than a viable number. It's a proposal sure to net a violent reaction from Senate Democrats, who have in recent years asked that defense increases be matched dollar-for-dollar on the non-defense side.

Democrats quickly signaled Monday they would oppose the plan. Senate Minority Leader Chuck Schumer, D-N.Y., said in a statement, "This budget proposal is a reflection of exactly who this president is and what today's Republican Party believes in: helping the wealthy and special interests while putting further burdens on the middle class and those struggling to get there."

Squeezing Trump and Mulvaney from the opposite side of the aisle, in the lower chamber, Thornberry said, "we can and should do more" for defense.

"While we cannot repair all of the damage done by those cuts in a single year, we can and should do more than this level of funding will allow," Thornberry said in a statement. "The administration will have to make clear which problems facing our military they are choosing not to fix. We cannot make repairing and rebuilding our military conditional on fixing our budget problems or on cutting other spending."

Katherine Blakeley, a defense budget expert with the Center for Strategic and Budgetary Assessments, says a number of questions need to be addressed before the real details of the budget plan can be assessed. But one thing is clear, she said – that this plan is designed to be "politically untenable for the Democrats while trying to be as favorable as possible for the Republicans."

Or as Benjamin Friedman of the CATO Institute put it, "This plan won't happen because Democrats will block it."

Since the imposition of the budget caps, Democrats have fought to maintain a one-for-one trade between defense spending and domestic spending. That was easier when they had top cover from President Barack Obama, but could prove more challenging with the Trump White House hammering Democrats for holding down on defense levels.

Congress would not only need to pass a 2018 budget resolution, which could pass with Republican alone, but a new law to raise the caps to allow the White House's proposal. That would require Democratic votes.

"Republicans need 60 Senate votes to overturn the BCA cap for next year," Friedman said. "I believe Schumer can keep enough of Democrats in line around the idea that an increase in defense must be matched by an increase in non-defense discretionary to stop this."

Instead, he predicts the two parties will settle on a raising of the budget caps for both defense and non-defense spending, with an influx of cash into OCO.

Blakeley also notes that, by putting its marker for fiscal 2018 down now, the administration is setting up the still-unsettled fiscal 2017 appropriations discussion as a proxy war. The government is currently operating under a continuing resolution to fund the federal government, which expires at the end of April. 

Mackenzie Eaglen, of the conservative American Enterprise Institute, said of Democrats, "This will really irk them and there's no reason for them not to go all in on this political battle. Cutting programs like Head Start and National Endowment for the Arts are not even popular among many Republicans."

Because the plans pays for defense increases through the non-defense side and not deficit spending, the proposal is a nod at fiscal conservatives in Congress. Mulvaney was one of them before he left Congress to join the White House.

"Nobody wants to increase overall spending, and the only way you can do that without huge earth-shaking debates is by reducing other discretionary spending," said James Jay Carafano, a national security expert at the conservative Heritage Foundation. "There is a rational limit to how much you can do that."

It is also a nod toward defense hawks. Carafano described the White House's number Monday as a "down payment" on the defense build-up Trump has repeatedly promised.

"The important thing to me is this is a clear statement of recognition that we are heading towards a hollow force," Carafano said. "There's not just a readiness crisis, there's enormous modernization challenges, and you have trouble maintaining [operational] tempo. That's the text book definition of a force that's heading to hollow."