President Donald Trump on Thursday dusted off the cobwebs of an old proposal from April to deploy military troops along the southern U.S. border in an effort to stem illegal crossings by immigrants.

“I will call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!” the president threatened, if Mexico does not heed his request to stop what he termed an “onslaught” of people crossing the border into the U.S.

Trump claimed that “Criminal elements and DRUGS [are] pouring in” and that it is “All Democrats [sic] fault for weak laws!”

I am watching the Democrat Party led (because they want Open Borders and existing weak laws) assault on our country by Guatemala, Honduras and El Salvador, whose leaders are doing little to stop this large flow of people, INCLUDING MANY CRIMINALS, from entering Mexico to U.S…..

— Donald J. Trump (@realDonaldTrump) October 18, 2018

….In addition to stopping all payments to these countries, which seem to have almost no control over their population, I must, in the strongest of terms, ask Mexico to stop this onslaught – and if unable to do so I will call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!..

— Donald J. Trump (@realDonaldTrump) October 18, 2018

….The assault on our country at our Southern Border, including the Criminal elements and DRUGS pouring in, is far more important to me, as President, than Trade or the USMCA. Hopefully Mexico will stop this onslaught at their Northern Border. All Democrats fault for weak laws!

— Donald J. Trump (@realDonaldTrump) October 18, 2018

The issue with Trump’s threat is that deploying active U.S. military troops for domestic law enforcement duties is illegal, unless he declares a national emergency and is granted a waiver by Congress.

After he issued a similar threat in April, the president’s administration was forced to issue a statement signaling that it was moving toward a plan to use the National Guard — not active duty forces — which would be more doable legally.

Such a move to buttress law enforcement efforts along the border with National Guard units has recent precedents.

President George W. Bush deployed the National Guard to assist in border security operations in 2006 and President Barack Obama did the same in 2010.

The 1878 Posse Comitatus Act bars Trump from using the military as a police force, and while Trump could ask Congress for a waiver, that is unlikely because Senate Democrats, who oppose his border security agenda, can block passage of most legislation.

Watch: Trump Heads West to Campaign — And a Lot of Senators Do, Too