President Donald Trump on Friday declared a national emergency to obtain funding for a wall along the US-Mexico border.

During his announcement, Trump said: "I didn't need to do this. I just want to get it done faster, that's all."

A border wall was one of Trump's biggest promises during his 2016 presidential campaign.

Trump is likely to face legal challenges to his emergency declaration, which Democrats say is an abuse of executive power.

The president's statement that he "didn't need to" declare the emergency could come up during any legal battles, according to some legal experts.

President Donald Trump on Friday declared a national emergency to get funding for a wall along the US-Mexico border and in the process of delivering a lengthy defense of his decision said, "I didn't need to do this."

"I just want to get it done faster, that's all," Trump added.

During his 2016 presidential campaign, Trump promised to build a wall along the border to help prevent illegal immigration and the influx of drugs from Mexico.

The president has struggled to make good on this pledge since entering the White House, and a fight with congressional Democrats over funding for a wall recently led to the longest government shutdown in US history.

Trump ended the shutdown in late January by signing a bill that temporarily funded the government, and he's set to sign a new funding bill on Friday.

Read more: Why a GOP congressman who represents more of the border than anyone in Congress opposes Trump's wall

The new bill includes $1.4 billion for physical barriers on the border, much less than the $5.7 billion the White House was seeking. The funding shortfall is part of Trump's justification for declaring the national emergency, something he had threatened for weeks.

Democratic leaders on Friday denounced Trump's declaration as "unlawful."

In a joint statement, House Speaker Nancy Pelosi and Minority Leader Chuck Schumer said: "The President's actions clearly violate the Congress's exclusive power of the purse, which our Founders enshrined in the Constitution. The Congress will defend our constitutional authorities in the Congress, in the Courts, and in the public, using every remedy available."

Read more: Several House Republicans broke with Trump and voted with Democrats to pass 2 bills that would end the government shutdown

Some prominent Republicans, such as Sens. Marco Rubio and Chuck Grassley, had expressed concerns about the precedent Trump sets by declaring a national emergency and warned that he would face opposition in court.

Trump alluded to that during his Friday announcement, saying he expects to be sued.

The president's statement that he "didn't need to" declare the emergency could come up during any legal battles, according to some legal experts.

National emergencies have often been declared to impose sanctions on people involved in human-rights abuses in various parts of the world, particularly Africa and Central America.