Fifteen states have joined California in filing suit to stop Donald Trump from using money from the military to build a border wall.

The attorneys general from Colorado, Connecticut, Delaware, Hawaii, Illinois, Maine, Maryland, Michigan, Minnesota, Nevada, New Jersey, New Mexico, New York, Oregon, and Virginia joined California in the lawsuit, which was filed in the U.S. District Court for the Northern District of California, perhaps the most liberal U.S. district court in the nation.

It's not unusual for leftists to go venue-shopping when they know they have a weak constitutional case to make.  The Northern District is located in San Francisco and would be expected to side with the blue states who filed suit. 

It hardly matters what the outcome of the case will be in lower courts, as the issue is almost certainly destined to land in the lap of the Supreme Court.

The core argument is that Trump is circumventing Congress to fund the wall by declaring an emergency.

CNN:

"The Constitution assigns Congress the power of the purse, and no prior president has ever tried to use emergency powers to fund a chosen project — particularly a permanent, large-scale domestic project such as this — against congressional will.  This is obviously improper," said Dror Ladin, staff attorney with the ACLU's National Security Project.

Becerra argued that the states have standing to challenge Trump because money appropriated to them might be at risk.

"If the President is essentially stealing money that's been allocated to go to the various states for various purposes but no longer will, we're being harmed, our people are being harmed," he said.

The wave of lawsuits was expected, though fighting them in court will likely be difficult.

Trump may ultimately prevail in the Supreme Court, which historically has granted the president much leeway in determining what is and what isn't a national emergency:

The National Emergencies Act allows the President to declare a national emergency and unlock a stash of funds by invoking certain statutory authority.  The President has wide discretion over what constitutes a national emergency.  As a result, legal experts argue that fighting the declaration on the basis of the emergency itself will likely be difficult.  The other question is whether the statute Trump has invoked — which in this case, requires the use of the armed forces — can be used to fund the wall.

Under the declaration, the administration will tap $2.5 billion of military narcotics funding and $3.6 billion in military construction funding.  Acting US Defense Secretary Patrick Shanahan said he will start studying which projects to pull from and determine whether border barriers are necessary to support the use of the armed forces.

One of the primary arguments of the states will be that Trump is stealing money from funds already appropriated to go to the states.  This will be extremely difficult to prove, as each individual state will have to show that specific funds from wherever the money is coming from were destined for it. 

But expect the liberal justices of the Northern District to gloss over the details and find some specious grounds to rule against Trump.

Congress may very well express its opposition to the emergency declaration, and Trump will promptly veto it.  Since there aren't enough votes in the House or Senate to override the president's veto, this avenue is a dry hole for emergency declaration opponents.

It's likely that an injunction will be issued to prevent Trump from implementing his emergency declaration.  That ruling would be challenged on constitutional grounds, as the courts would be violating the separation of powers by meddling in the business of the executive.

Most legal experts believe that the president is on solid ground in declaring the emergency.  Whether the courts will agree is another question.