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(CNN) —  

President Donald Trump has become increasingly sensitive to criticism that he’s backing off his signature promise to build a wall along the US-Mexico border, three sources familiar with his concern tell CNN, as aides fear the administration’s chances for securing funding for it have sunset.

GOP lawmakers and Trump are set to OK a short-term funding measure to avert a year-end government shutdown and Democrats will assume control of the House in 2019, virtually assuring no additional border wall funding.

Trump’s anxiety about fulfilling his top campaign goal comes at the end of months of bitter debate inside the West Wing over how to fund the border wall between those seen as ideologues and those who consider themselves pragmatists.

White House officials were in a meeting discussing border security last August when a sudden outburst from aide Stephen Miller silenced the room. The President’s legislative affairs director, Shahira Knight, was in the middle of arguing that instead of pushing for the border fight so close to the midterm elections, the White House should hold off until after November when they would be in stronger position to fight over funding.

Miller, the immigration hardliner at the table, cut her off mid-sentence. According to two people in the room, he shot back with a list of reasons why the administration would almost certainly be in a weaker position after the elections because Republicans were guaranteed to lose multiple seats and possibly the entire House, making building the wall all but impossible.

Knight, a former aide to ex-White House economic adviser Gary Cohn with an in-depth policy knowledge, went quiet. An official described it as an “evisceration.”

The exchange encapsulates how the President’s aides have intensely disagreed on the best way to accomplish his aims, including the border wall, and whether some of his goals are worth the fight.

The President launched his 2016 presidential campaign on immigration, tapping into a rock solid base of support that catapulted him into the White House. He has relied on immigration fears – and promises of security – during campaigning, leaning heavily on the issue leading up to the midterm elections. Though some political observers in Washington scoffed at his promise to build a wall and have Mexico pay for it, his base fully supported it. But recent events have shown that the President could be facing a major defeat on his signature promise, leading him to lash out.

Trump postponed the summer fight over the border wall when he signed a short-term spending bill in September, but the issue has resurfaced and the same disagreements that were laid bare in that August meeting still exist. The prospect of funding the wall this time around grew dim when Senate Majority Leader Mitch McConnell announced Wednesday he will introduce a continuing resolution, a deal that will fund the government through February 8 but deny the President his wall funding.

In a radio interview Wednesday, Ann Coulter, a conservative who has criticized the lack progress on the wall, declared she won’t vote for Trump in 2020 if the wall isn’t built.

“They’re about to have a country where no Republican will ever be elected president again,” Coulter told the station WMAL. “Trump will just have been a joke presidency who scammed the American people, amused the populists for a while, but he’ll have no legacy whatsoever.”

It remains to be seen if his supporters will chalk this up as a loss. Trump has continued to pledge he will get his “big, beautiful wall.”

Despite aides signaling an impending concession on the border wall, the President outwardly insisted Wednesday that the wall will be built “one way or the other.”

“Mexico is paying (indirectly) for the Wall through the new USMCA, the replacement for NAFTA! Far more money coming to the U.S. Because of the tremendous dangers at the Border, including large scale criminal and drug inflow, the United States Military will build the Wall!” he tweeted.

And Trump has publicly embraced the idea of shutting down the government if he doesn’t get funding for the border wall, declaring in front of cameras in the Oval Office that he would be “proud” to do so.

White House officials have been hesitant to guarantee the President would sign the measure McConnell proposed Wednesday, but aides have signaled a softening in their position this week.