Saying he is "not very optimistic" about a deal being reached over border wall funding before a Feb. 15 deadline, Sen. Lindsey Graham, R-S.C., urged President Donald Trump to "go it alone" and declare a national emergency to get the wall built, The Post and Courier reported Monday.

Trump gave negotiators three weeks to arrive at an agreement before funding for parts of the federal government run out again, setting up another potential shutdown.

Sen. Graham said the fate of the border would be the "defining moment" of Trump's presidency. Trump campaigned on the issue in 2016, promising repeatedly that Mexico would pay for the wall.

"I'll be disappointed if we don't get money for the wall," Graham told reporters.

But he advised against another shutdown, saying, "The best way to do this, if we can't get a deal with Congress, is for the president to take money that's already appropriated for national security, unobligated, and apply it to a border barrier . . . [because] I think shutting down the government does not enhance the cause of border security."

Graham acknowledged the debate could start a "war within the Republican Party," with some in the GOP saying declaring a national emergency should not be considered.

A main concern among some Republicans, in addition to the problem of bypassing Congress, is how a future Democratic president would use emergency powers if Trump sets a precedent of using it for what critics say is a political goal and not a real emergency.

Those who are against Trump using his emergency powers include Senate Majority Leader Mitch McConnell, R-Ky., who told the president it could result in Congress voting to disapprove of his declaration, The Washington Post reported.

But Graham insists "this is within the power of every commander-in-chief" and called on his Republican colleagues to stick together, labeling the wall the "defining moment" of Trump's presidency.

Under a 1976 law, Trump can declare a national emergency if he specifies the reason to Congress. But any lawmaker could demand a vote on overturning the president's decision, setting up a test of the president's power and his pull within the GOP. If Congress disapproves, though, the president has the ability to veto such a resolution.

Graham does expect the president to follow his advice and order the military to build the wall using money in national security accounts. He said the past three presidents sent troops to the border and Graham suggested that building a wall is not much different.

Graham is up for re-election in 2020 in a state Trump won by 14 percentage points. He denied he has changed his political views to defer to the president, who has become a golfing buddy.

"'What's happened to Lindsey Graham?'" he said someone from New York asked him. "'Not a damn thing.'"

Disagreement over border wall funding led to a 35-day partial government shutdown in December and January which ended after Trump agreed to fund the government for three weeks while lawmakers try to reach a deal that would satisfy his demands.

Trump has said there is a "good chance" he will have to declare a national emergency, which would allow him to circumvent Congress and repurpose funds appropriated for other purposes in order to build the wall.

Material from Bloomberg news service was used in this story.