WASHINGTON – President Trump said Thursday he plans to sign an order next week that could lead to the large-scale detention of migrants crossing the southern border and bar anyone caught crossing illegally from claiming asylum – two legally dubious proposals that mark his latest election-season barrage against illegal immigration.

“This is an invasion,” Trump declared as he has previously on a subject that has been shown to resonate strongly with his base of Republican supporters. He made his comments at the White House in a speech that was billed as a response to groups of migrants currently walking toward the U.S. border.

He then left for an evening political rally in Missouri, the latest in a daily series he has scheduled leading up to next Tuesday’s elections for control of Congress.

U.S. immigration laws make clear that migrants seeking asylum may do so either at or between border crossings. But Trump said he would limit that to official crossing points. The U.S. also doesn’t have space at the border to manage the large-scale detention of migrants, with most facilities at capacity. But Trump said the government would erect “massive tents.”

“We’re stopping people at the border,” he said firmly.

His announcement marked Trump’s latest attempt to keep the issue of immigration front-and-center in the final stretch before next Tuesday’s elections. Notably, he said his executive order would come next week, which means it could be after Election Day.

Trump also said that he had told the U.S. military mobilizing at the southwest border that if U.S. troops face rock-throwing migrants, they should react as though the rocks were “rifles.”

The exact rules for the use of force by military police and other soldiers who will be operating near the border have not been disclosed, but in all cases troops have the right of self-defense. At any rate, they are not expected to be in positions where anyone trying to storm across the border would quickly come in contact with them.

Mark Hertling, a retired Army general, wrote on Twitter after Trump’s speech that no military officer would allow a soldier to shoot an individual throwing a rock. “It would be an unlawful order,” he wrote, citing the Law of Land Warfare.

The president announced on Wednesday that he was considering deploying up to 15,000 troops to the U.S.-Mexican border in response to the still far-off caravans – roughly double the number the Pentagon said it currently plans for a mission whose dimensions are shifting daily.

Trump said Thursday that, under his order, any migrants who do manage to enter the country illegally will be housed in “massive tent cities” he plans to build while their cases are processed.

“We’re not releasing them into our country anymore,” he said. “We have thousands of tents.”

Under current protocol, many asylum seekers are released while their cases make their way through back-logged courts – a process that can take years.

Trump and other administration officials have long demanded that those seeking asylum come through legal ports of entry. But many migrants are unaware of that guidance, and official border crossings have grown increasingly clogged. Immigration officials have turned away asylum-seekers at ports of entry because of overcrowding, telling them to return at a later date. Backlogs have grown especially bad in recent months at crossings in California, Arizona and Texas, with people generally waiting five weeks to try to claim asylum at San Diego’s main crossing and sleeping out in the open for days at a time.

The administration has also been ramping up security at ports of entry this week. In McAllen, Texas, in the Rio Grande Valley, workers were seen installing additional gates and fences along a walkway on a bridge between the U.S. and Mexico, according to The Monitor newspaper of McAllen.

Migrants who cross illegally are generally arrested and often seek asylum or some other form of protection. Claims have spiked in recent years, and there is currently a backlog of more than 800,000 cases pending in immigration court. Administration officials have railed against what they say are loopholes designed to encourage people, especially from Central America, to come to the U.S. and claim asylum. Generally, only about 20 percent of applicants are approved.

The U.S. fielded more than 330,000 asylum claims in 2017, nearly double the number two years earlier and surpassing Germany as highest in the world.

There are currently four caravans making their way toward the U.S. The main group of about 4,000 migrants – down from its estimated peak of more than 7,000 – remains in southern Mexico, mostly on foot and hundreds of miles from the border. A second, smaller group of 1,000 or so is more than 200 miles behind the first. A third band of about 500 from El Salvador has made it to Guatemala, and a fourth group of about 700 set out from the Salvadoran capital Wednesday.

Trump has nonetheless mounted an enormous show of force in response.

The first 100 active duty troops arrived at the border in McAllen on Thursday – part of the “more than 7,000” troops the Pentagon said were being sent to support Customs and Border Protection agents.

But troop estimates have been changing at a dizzying pace. Just last week officials were indicating that about 800 to 1,000 might be sent. On Monday, officials announced that about 5,200 were being deployed. The next day, the Air Force general running the operation said more than the initially announced total were going, but he pointedly rejected a news report that it could reach 14,000, saying that was “not consistent with what’s actually being planned.”

Just 24 hours later, Trump caught the Pentagon by surprise.

“We have about 5,800. We’ll go up to anywhere between 10,000 and 15,000 military personnel on top of Border Patrol, ICE and everybody else at the border,” he said Wednesday.

Trump has rejected the idea he has been “fearmongering” and using the issue for political purposes, but his escalating rhetoric in the final days of the campaign calls that denial into question. He also has said he plans an executive order to unilaterally end the constitutionally protected right of citizenship for children born to non-U.S. citizens.

Trump also tweeted a video Wednesday alleging Democrats were responsible for allowing a homicidal immigrant into the U.S. but provided no evidence supporting that claim.

The video includes scenes of migrants moving toward the U.S. and asks ominously, “Who else would Democrats let in?”

It was reminiscent of the infamous “Willie Horton” ad used against Democratic presidential candidate Michael Dukakis in 1988 and condemned as racist. Horton, who was black, raped a woman while out of prison on a weekend furlough. As Massachusetts governor, Dukakis supported the furlough program. He lost to Republican George H.W. Bush.

Associated Press writers Elliot Spagat in San Diego, Amy Taxin in Santa Ana, California, and Zeke Miller and Robert Burns in Washington contributed to this report.