The Department of Defense "has no legal authority" to use appropriated funds for President Donald Trump's border wall between the U.S. and Mexico, two Democratic senators wrote in a letter to Defense Secretary James Mattis.
"Until we can have a wall and proper security we're going to be guarding our border with the military," Trump said Monday at the White House. "That's a big step."
Trump originally floated the idea of the military paying for the wall over Twitter and in a discussion with Mattis late last month.
"Such a controversial move could only be funded by cutting other vital priorities for our service members, mere weeks after the Department communicated its needs to the Senate Defense Appropriations Subcommittee during omnibus appropriation negotiations," Democratic Sens. Jack Reed and Dick Durbin wrote.
"We conclude that the Department of Defense has no legal authority, with or without a reprogramming request, to use appropriated funds for the construction of a border wall."
The letter from Reed, of Rhode Island, and Durbin, of Illinois, comes on the heels of reports that Trump suggested the idea to "several advisors" after being disappointed by the amount of money allocated for border security in the $1.3 trillion omnibus.
Dana White, the Pentagon's chief spokesperson, confirmed last week that Mattis had "initial" talks with Trump about diverting a portion of the DOD's budget to construct a wall on the border.
The Pentagon did not immediately respond to CNBC's request for comment.
Congress would have to appropriate the money to make it work — a notion Sen. Tim Scott, R-S.C., touched on over the weekend. Scott said that Congress would "absolutely" need to weigh in on reprogramming defense funds to the border wall.
"The reality of it is, as commander-in-chief, he can certainly send signals through Secretary Mattis, have a conversation with Congress about where those dollars should be spent," Scott said told CBS' "Face the Nation" on Sunday.
Scott, a member of the Senate Armed Services Committee, touted the border wall as a matter of national security but noted that the Department of Defense may not be the best organization to construct it.
"The unfortunate reality is that the priorities of the DOD have already been set. However, our southern border is very porous. The truth is that more folks came through our southern border that did not come from Mexico. It is certainly a national security issue," Scott said.
Earlier this month, Trump signed the colossal spending bill into law, praising it as a "matter of national security." His signature on the 2,200-page legislation grants the most significant increase in defense funding in the past 15 years. The Pentagon is set to gain $61 billion more than last year's enacted funding for a top line of $700 billion.
Trump wanted as much as $25 billion to build a wall between the U.S. and Mexico, but the legislation only authorized $1.6 billion for fencing, surveillance technology and other measures. The president campaigned in large part on building a wall and saying that Mexico would pay for it.
In his first public appearance since signing the defense-friendly bill, Trump told a crowd in Ohio last week that he has not given up on the wall.
"That's what I do, I build, I was always very good at building. It was always my best thing. I think better than being president I was maybe good at building," Trump said Thursday.
During the presidential campaign, Trump repeatedly promised Mexico would fund the wall, but he has backed off those pledges in office, seeking federal money.
Below is the full text of the senators' letter: