Donald Trump is prepared to issue the first veto of his presidency over a national emergency declaration his critics have slammed as an unconstitutional power grab, a senior White House official said on Sunday.

White House senior adviser Stephen Miller told Fox News Sunday that “the president is going to protect his national emergency declaration”.

Asked if that meant the president was ready to veto, Mr Miller added: “He’s going to protect his national emergency declaration, guaranteed.”

The president declared the emergency Friday in an effort to go around Congress to fund his border wall. It would allow him to move federal dollars earmarked for military construction to the border – but is already facing legal and political challenges.

The proclamation cited an increase in families coming across the border and an inability to detain families during deportation proceedings – not drugs or violence as the president outlined in his press conference. The top two Democrats in Congress said they’d use “every remedy available” to oppose what they cast as an unlawful measure.

The Independent goes on a ride-along with US Border Patrol Show all 15 1 /15 The Independent goes on a ride-along with US Border Patrol The Independent goes on a ride-along with US Border Patrol Texas and Mexico are connected by entry points like these. US Border Patrol in Laredo, Texas conducts daily patrols throughout the Rio Grande, snaking between the US and Mexico, where it searches for migrants and drug traffickers. The legal entry point to Mexico can be seen above the riverines frequently used by cartels to funnel narcotics into the US. Chris Riotta The Independent goes on a ride-along with US Border Patrol US Customs and Border Protection oversees its side of the entry point. US Customs and Border Protection officers oversee the processing of nearly two million trucks and three million pedestrians annually at the Laredo sector, which also processes the majority of trade between the US and Mexico. Chris Riotta The Independent goes on a ride-along with US Border Patrol The Rio Grande snakes between Mexico and the United States. The US and Mexico have bordering coastlines in Laredo Texas, where the Rio Grande's occasionally rough currents splash onto both nations' shores. For migrants attempting to enter into either country, the swim is much more dangerous than it appears. Chris Riotta The Independent goes on a ride-along with US Border Patrol Drug traffickers and migrants use "highways" to cross the border. US Border Patrol agents call worn pathways like these "highways," as they are frequently used by drug traffickers and migrants entering into the US. Chris Riotta The Independent goes on a ride-along with US Border Patrol The journey is still far from over. If they're able to make it onto land, many obstacles still remain for migrants and drug traffickers. US Border Patrol agents are equipped with high-tech security, K-9 units and aerial camera footage surveilling 40 miles of borderlands. Chris Riotta The Independent goes on a ride-along with US Border Patrol Border Patrol uses classic methods and high-tech to surveil the border. A control room at the Laredo sector headquarters is fully-manned 24/7 with a team of local employees who alert agents on the ground of border crossings in real-time. Their cameras live-stream 40 miles of the 170 mile territory the sector covers. Chris Riotta The Independent goes on a ride-along with US Border Patrol The US provides trusted traveler programs on both its southern and northern borders. Officers lead the way to Sentri offices, which runs a trusted traveller program from US and Mexican local residents who regularly cross the border for work or leisure purposes. Chris Riotta The Independent goes on a ride-along with US Border Patrol The pathway to freedom. Once travellers are processed, they walk up this ramp towards the US. Many Mexicans cross the border each day with collapsable shopping carts to buy goods while visiting the country. Chris Riotta The Independent goes on a ride-along with US Border Patrol But first, the wait. A dog waits for their owner to go through processing with US Customs and Border Patrol. On a good day, processing can take under an hour. On a bad day, well, there's just no telling how long one might be stuck inside the Laredo sector field offices. Chris Riotta The Independent goes on a ride-along with US Border Patrol The first lady of CBP. Inside the Laredo field offices, US Border Patrol agents keep a framed photo of Christine Davis, the first female agent to join the federal agency in 1975. Chris Riotta The Independent goes on a ride-along with US Border Patrol "The hall of fame." US Border Patrol also honors those who have served in the agency for over thirty years. Chris Riotta The Independent goes on a ride-along with US Border Patrol Officers inspect a vehicle in secondary processing. Outside, officers are conducting secondary inspection on a vehicle that's been pulled aside for further investigating. Chris Riotta The Independent goes on a ride-along with US Border Patrol The waiting room for detained migrants. A US Customs and Border Protection officer shows the inside of a holding room where people requiring further questioning go through secondary processing. Chris Riotta The Independent goes on a ride-along with US Border Patrol A man and his daughter go through processing. US Customs and Border Protection also regularly provides humanitarian visas and other temporary permissions for people with valid reasons seeking entry to the US. Here, a father and his daughter await to see whether they will be provided one of those visas. Chris Riotta The Independent goes on a ride-along with US Border Patrol Laredo processes over three million pedestrians annually. Thousands of legal asylum seekers, migrants and refugees seek entry to the US through the southern border annually, where they go through processing at offices like this after arriving at a legal port of entry. For many, the trip can take days, if not weeks or even months. Chris Riotta

California Attorney General Xavier Becerra said his state would sue “imminently” to block the order, after the American Civil Liberties Union and the nonprofit watchdog group Public Citizen announced Friday they were taking legal action.

Democrats are planning to introduce a resolution disapproving of the declaration once Congress returns to session that is likely to pass.

Several Republican senators are already indicating they would vote against Mr Trump – though there do not yet appear to be enough votes to override a veto by the president.

In his emergency proclamation, Mr Trump painted a dark picture of the border as “a major entry point for criminals, gang members, and illicit narcotics” and one that threatens “core national security interests”. Overall, though, illegal border crossings are down from a high of 1.6m in 2000.

His declaration instantly transformed a contentious policy fight into a foundational dispute over the separation of powers enshrined in the constitution, spurring talk of a congressional vote to block Mr Trump and ensuring that the president and Democrats will continue fighting over the border wall in Congress, the courts and on the campaign trail.

It triggered outrage from Democrats, unease among some Republicans and flew in the face of years of GOP complaints that Barack Obama had over-reached in his use of executive authority.

“By the president’s very own admission in the Rose Garden, there is no national emergency. He just grew impatient and frustrated with Congress, and decided to move along his promise for a border wall ‘faster,’” said American Civil Liberties Union executive director Anthony Romero.

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Republicans had opposed the president declaring a national emergency, repeatedly warning that it would set a bad precedent and divide the party when Democrats put it up for a vote. While many in the GOP on Friday fell in line behind Mr Trump’s decision, others remain opposed.

“I don’t believe a national emergency declaration is the solution,” Thom Tillis, a North Carolina Republican, said in a statement.

“It wouldn’t provide enough funding to adequately secure our borders, it would likely get tied up in litigation, and most concerning is that it would create a new precedent that a left-wing president would undoubtedly utilize to implement their radical policy agenda while bypassing the authority of Congress.”