Sen. Lindsey Graham, R-South Carolina, said that President Trump has "got to do it on his own" to build a border wall in an interview with "Face the Nation" Sunday. Mr. Trump — dissatisfied with the $1.375 billion Congress provided him for barriers at the southern border — declared a national emergency to free up more funds to construct border barriers on Friday.

"Unfortunately, when it comes to Trump, the Congress is locked down and will not give him what we've given past presidents," Graham said. "So unfortunately, he's got to do it on his own, and I support his decision to go that route."

Graham, a staunch ally of Mr. Trump in the Senate, noted that former Presidents George Bush and Barack Obama sent troops to the border. He argued that many in Congress only oppose the wall because Mr. Trump is the person proposing it.

Graham also discussed Mr. Trump's comment Friday that he "didn't need to" call the national emergency, but he did so because he would "rather do it much faster." Graham said that this comment did not open the national emergency to legal challenges.

"I think the president's been making a persuasive case that those borders [are] broken. Drugs are flowing across the border killing Americans, [there is] human trafficking. We've got a dangerous situation along the border," Graham said. He added that the president has the authority to send troops to the border and build barriers while they are deployed.

"I support his desire to get it done sooner rather than later. And I'm disappointed that my Democratic colleagues would not give the President the money to secure the border that they were willing to give the Bush and Obama," he said.

However, the president's decision to declare a national emergency is also facing intense criticism from some Republicans. Tennessee Sen. Lamar Alexander released a lengthy statement calling the declaration "unnecessary, unwise and inconsistent with the U.S. Constitution" — "unnecessary" because Congress had appropriated funds for border security, "unwise" because future presidents could misuse the power, and "inconsistent" with the Constitution because only Congress is bestowed with the power to tax and spend the people's money.

Democratic attorneys general and groups like the American Civil Liberties Union (ACLU) have threatened to file suit against the declaration. House Speaker Nancy Pelosi didn't rule out a legal challenge on Thursday. Mr. Trump predicted he will be sued but will ultimately prevail in the Supreme Court.

Mr. Trump is expecting to use $8 billion to build the wall, including the $1.375 billion approved by Congress, with an additional $600 million expected to come from the Treasury Department's drug forfeiture funds, $2.5 billion coming from the Defense Department's drug interdiction program, and an additional $3.5 billion coming from the Pentagon's military construction budget.