President Trump announced Friday that he would declare a national emergency to build his proposed Mexico border wall, citing crime and violence as his rationale for a move that allows him to redirect funds toward his top 2016 campaign priority.

"We're declaring it for virtual invasion purposes," Trump said in a Rose Garden speech."It's not like it's complicated, it's very simple. We want to stop drugs from coming into our country. We want to stop criminals and gangs from coming into our country.

White House Acting Chief of Staff Mick Mulvaney shared details on a media call before Trump's speech, saying the declaration would allow $8 billion for border barrier construction, including the $1.37 billion in fencing funds approved by Congress this week. Trump signed that spending bill into law at around 2 p.m. Friday, after his public remarks.

"We have to do it. Not because it was a campaign promise, which it is. It was one of many, by the way," Trump said. "It's a great thing to do."

Trump's largely unscripted remarks featured a reference to mothers of children killed by illegal immigrants, one of whom he asked to show a photo of her child.

Many portions of Trump's opening remarks — he also took questions from reporters — were recycled from his two-month push for Democrats to approve wall funds, including his argument that human smugglers chose unwalled sections of the border to transport "women with tape on their mouth."

Trump acknowledged he was facing a drawn-out court battle but predicted victory in the land's highest court.

"I'll sign the final papers as soon as I get into the Oval Office," he said. "We will have a national emergency. We will then be sued and they will sue us in the 9th Circuit, even though it shouldn't be there. We will possibly get a bad ruling, we'll get another bad ruling and we'll end up in the Supreme Court, and hopefully we'll get a fair shake and we'll win in the Supreme Court," he said.

The legal process could take mere months under expedited procedures, experts say.

Presidents have declared dozens of other emergencies since 1976 for reasons ranging from swine flu to countering "blood diamonds," 31 of them still in effect, but none of the declarations attracted much controversy. Trump is likely to face lawsuits from Democrats, environmentalists, and landowners.

A senior administration official on the call with Mulvaney said an emergency declaration was needed only to redirect $3.6 billion in military construction funds, saying low-priority projects would be shelved. The official said Trump could redirect without an emergency about $600 million from the Treasury Department's forfeiture fund and about $2.5 billion from Defense Department anti-drug activities.

The senior official said the Trump administration plans to built 234 miles of steel bollard wall along the 1,954-mile U.S.-Mexico border but did not specify where. The projected length is the same as what was originally sought when Trump requested $5.7 billion in wall funds, prompting a 35-day partial government shutdown in December and January.

"We assess that with the $8 billion we should have sufficient money this year to do what we wanted to do with the [$5.7 billion] worth of money that the president asked for originally," Mulvaney said.

"We continue to look for pots of money that can be used for this. We have not ended that," the other official said, saying the Trump administration will seek to replace military funds with March budget requests.

"We identified more than the [$5.7 billion] to make sure we have enough money."

[Opinion: Texas border agents keep catching rapists, child molesters, and gang members, but Democrats don’t care]