Democrats and campaign groups have vowed to fight Donald Trump’s “unlawful” national emergency declaration as he revealed plans to raid existing government funds to build his Mexico border wall.  The US president hopes to gather $8 billion through various means to construct barriers along America’s southern border, declaring at a White House press conference that “walls work 100 per cent”.  Most controversial is the $3.6 billion Mr Trump wants to redirect from existing military construction projects by using a power only available to him after declaring a national emergency. He also plans to take $2.5 billion from the Pentagon’s anti-drugs fund and $600 million from money forfeited to the Treasury. A further $1.375 billion comes from legislation agreed by Congress in a compromise spending deal.    “We’re going to confront the national security crisis on our southern border and we’re going to do it one way or the other,” Mr Trump said. He added: “We have an invasion of drugs and criminals coming into our country.”   Donald Trump will no doubt face legal challenges  Credit: AP However there was a fierce backlash from political opponents, advocacy groups and even some Republican senators who believe the move amounts to constitutional overreach and could be defeated in the courts.  Nancy Pelosi and Chuck Schumer, the Democrat leaders in the House of Representatives and Senate respectively, issued a statement condemning the announcement.  “The President's unlawful declaration over a crisis that does not exist does great violence to our Constitution and makes America less safe, stealing from urgently needed defense funds for the security of our military and our nation,” the joint statement read.  "This is plainly a power grab by a disappointed president, who has gone outside the bounds of the law to try to get what he failed to achieve in the constitutional legislative process." The Democrats are yet to decide if they will launch their own legal action but campaign groups including Protect Democracy, a body run by former government lawyers, and the Niskanen Center, a liberal think tank, have vowed to do so.   More than half a dozen Republicans senators also publicly criticised the move, calling it a “bad idea”, a “mistake” and “unnecessary”.  Some could even join Democrats in backing a resolution blocking the declaration, though ultimately that needs the backing of two-thirds of all senators to be binding, which seems unlikely.  Mr Trump was bullish about a legal battle, predicting that his government would be sued but that the Supreme Court would eventually rule in his favour.  There is no set definition of what amounts to a national emergency, with the president able to call one when he wants. Doing so theoretically opens up more than 100 statutory powers which can be used.  However each has a strict legal definition. The Trump administration will have to prove how raiding from existing military construction budgets to build the border wall is permissible under the law.  National Emergencies | When and how they've been used in the past Mr Trump took the move after almost two months of discussions in Congress, including a 35-day government shutdown, produced enough funding for just 55 miles of border fencing.  Under the president’s new plan, 234 miles of wall made from steel bollards will be constructed along the border. Around 700 miles of the 2,000-mile border already has a barrier of some form.  None of the money will be taken from disaster relief funds for Puerto Rico or Texas, as had been speculated. It is also unclear where or when the barriers will be constructed.  A senior US official denied any dangerous precedent had been set by the move, as Democrats have claimed. "It’s not as if he just didn’t get what he wanted so he’s waving a magic wand and taking a bunch of money," the official said.  During a press conference about the announcement, Mr Trump also bemoaned the fact that he was unlikely to with the Nobel Peace Prize when discussing his upcoming North Korean summit.  The president also said that Shinzo Abe, the Japanese prime minister, had nominated him for the award, writing a “beautiful” five-page letter to the body that makes the decision.