President Trump's plan to send National Guard to the Mexico border is in motion, but the White House isn't offering many details

President Donald Trump's plan to deploy National Guard troops to the U.S. border with Mexico is in motion, with deployments coming as soon as Wednesday night, but otherwise, the administration has few details about the action.

A senior administration official, who spoke on condition of anonymity, reiterated much of what the administration had already announced about the plan but did not disclose how much it would cost or how long the mission would last.

When asked if there was a plan in place, a senior administration official said the discussion was by no means new.

"There have been ongoing conversations between DHS, DoD and the border states in regard to the mission that we will be supporting," the official said. "We are not going to DoD and saying we need X number of people; we are going to DoD and saying we need to fulfill this mission requirement, and DoD is working to identify and task that out."

The official also sought to head off any potential legal arguments against the plan.

"The best lawyers in the world are working on this and there is a lot of statutory language over a long period of time governing the National Guard and the issue of border security," the official said. "And lots of different options are available in terms of lawful and appropriate ways to utilize the National Guard and other state and federal resources to secure the border."

Trump has been pushing for more stringent border security measures since he signed a $1.3 trillion omnibus spending bill last month. He had been seeking $25 billion to build a wall on America's border with Mexico, but the bill included $1.6 billion for limited border security measures.

Members of his administration worked to respond quickly to the president's demands.

Homeland Security Secretary Kirstjen Nielsen announced Wednesday that her department, along with the Pentagon and border states' governors, would work together to deploy National Guard forces at the border.

"Border security is national security. It's a foundational element to the very security of our systems, of our citizens, and of our homeland," she said. "The president is committed to securing our border and using all the tools that he has available to him."

The Pentagon did not immediately respond to CNBC's request for comment.

Nielsen also said that the White House had put together legislation on border security.

"The administration has drafted legislation and we will be asking Congress again for the legal authority and resources to address this crisis," she said during a press briefing at the White House. "Based on conversations with congressional leadership, I am optimistic."

Attorney General Jeff Sessions, who has been a frequent target of Trump's ire, added his voice to the administration chorus calling for tougher border security. He noted the Justice Department's hiring of immigration judges but called on Congress to do more.

"This will not be enough if Congress does not act to pass clear, fair, and effective legislation that ends the illegality and creates a system that serves the national interest is crucial at this time," Sessions said in a statement Wednesday. "It is essential for Congress to act."