President Trump couldn't convince Congress to sign off on a $25 billion request to build a wall along the U.S. border with Mexico and efforts to get the Pentagon to pick up the tab may be hitting another kind of wall.

Pentagon press secretary Dana White told reporters Thursday that Secretary of Defense James Mattis spoke with Trump about the request this week.

"The secretary has talked to the president about it," White said at a regularly scheduled briefing. "Securing Americans and securing the nation is of paramount importance to the secretary."

White said she couldn't offer any additional specifics but budget and military spending experts say the unusual request might not be legal — at least not this year. Complex budget and spending rules limit how far government agencies can stray from strict funding guidelines included in spending bills.

Once Congress approves a spending measure and the president signs the bill into law, the spending levels are locked in until the end of the fiscal year. The spending bill that passed last week locks in spending levels through the end of September.

Every federal department has some flexibility to shift money around as their needs change. The rules for the Department of Defense are the most generous of the bunch. That extra wiggle room doesn't go far enough to cover the huge cost of building a border wall, according to Todd Harrison, a defense budget expert at the Center For Strategic and International Studies.

"Money is appropriated by Congress for a certain purpose and with certain constraints," Harrison said. "Once its appropriated, you have to use it for what it was appropriated for."

Harrison said the military can move smaller amounts, in the range of several million dollars, from program to program using what is known as reprogramming authority. But Congress has to sign off on any major funding shift.

"You can't do $25 billion," Harrison said. "They need to use that authority every year because sometimes money winds up being in the wrong place. You wouldn't want to be burning up much, if any, of your money on the wall."

Congressional approval isn't easy to obtain. The chairmen and ranking members on the House and Senate Armed Services Committees and the House and Senate Defense Appropriations Subcommittees would all have to sign off on the transfer. That means four Democrats and four Republicans would all have to agree to cut $25 billion from somewhere else in the Pentagon budget to pay to build the border wall.

House and Senate aides also poured cold water on the plan. There is no legal way for the Department of Defense to make that kind of transfer without new legislation from Congress, according to one of those aides.

The plan also faces major political hurdles — even within the Republican Party.

The spending bill set aside nearly $660 billion for the Pentagon — a roughly $61 billion increase from fiscal year 2017.

Republican leaders celebrated that increase as the biggest boost in defense funding in 15 years. Shifting $25 billion to the wall would make a huge dent in that increase.

That amount of money would be almost enough to buy two more of the$13 billion aircraft carriers Trump unveiled last year.It nearly matches the $25.4 billion increase to "procure, replace and upgrade" platforms and weapons included in the spending bill.

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