During his 2016 campaign, Trump promised to build a wall along the Mexico–United States border for which Mexico would pay. The president of Mexico rejected the idea of providing any funding for a U.S. border wall. In 2018, Trump requested $18 billion
in federal funding for some 700 miles (1,100 km) of barrier on the border, mostly to replace 654 miles (1,053 km) of aging fence built under the Secure Fence Act of 2006. On December 25, 2018, Trump reversed course, suggesting that he might accept 500 to 550 miles (800 to 890 km) of either mostly refurbished barrier (rather than new barriers in locations that did not previously have them) by November 2020. Trump's proposals and public statements on the wall have shifted widely over time, with varied proposals as to the design, material, length, height, and width of a wall.
In September 2018, Congress passed two "minibus" appropriations bills for the fiscal year 2019 federal budget, which began on October 1, 2018. These bills combined five of the 12 regular appropriations bills covering 77% of federal discretionary funding, and included a continuing resolution until December 7 for the remaining agencies. On December 6, Congress passed a second continuing resolution to December 21, to give more time for negotiations on Trump's proposed border wall, which had been delayed due to the death and state funeral of George H. W. Bush.
A Senate Homeland Security appropriations bill, negotiated by both parties and reported by the committee to the Senate, provided for $1.6 billion for border security, including funds for "approximately 65 miles of pedestrian fencing along the southwest border in the Rio Grande Valley Sector". The bill did not receive a vote on the Senate floor, although House Democratic whip Steny Hoyer indicated that such a proposal could be acceptable to House Democrats. Senate Minority Leader Chuck Schumer (D-NY) says the Democratic Party will not support $5.7 billion for the border wall. At a press conference before the government shutdown, he notes “The $1.6 billion for border security negotiated by Democrats and Republicans is our position. We believe that is the right way to go. …” 
Beginning of shutdown
Donald Trump meets with Nancy Pelosi and Chuck Schumer on December 11, 2018, stating "I will be the one to shut it down."
Overlook in San Francisco, closed for the shutdown in December 2018
On December 11, Trump held a televised meeting with Speaker-designee Nancy Pelosi and Senate Minority Leader Chuck Schumer in the Oval Office and asked them to support an appropriation of $5.7 billion for funding of a border wall. They refused, resulting in an argument between Trump and both Congressional leaders. During the contentious discussion, Trump said, "I am proud to shut down the government for border security ... I will be the one to shut [the government] down. I'm not going to blame you for it ... I will take the mantle. I will be the one to shut it down." Schumer replied, "We shouldn't shut down the government over a dispute."
Three days later, Politico reported that Trump was willing to sign a bill with no funding for a border wall that delayed a government shutdown into 2019 and the new Congress. On December 18, following a meeting with Trump, Senate Majority Leader Mitch McConnell said that the government would not shut down on December 22 and that Trump was "flexible" over funding for a border wall. Senate Appropriations Committee Chairman Richard Shelby commented that the most likely resolution was a bill that funded the government until early February. Schumer added that his caucus would "very seriously" consider such a bill and Senate Majority Whip John Cornyn said "I don't know anybody on the Hill that wants a shutdown, and I think all the president's advisers are telling him this would not be good."
The next day, the Senate unanimously passed a second continuing resolution (H.R. 695) that would fund the government until February 8, 2019. Pelosi announced that House Democrats would support the measure, meaning it would overcome opposition from conservative Republicans and pass the House. On December 20, following increased criticism from conservative media, pundits, and political figures, Trump reversed his position and declared that he would not sign any funding bill that did not include border wall funding. The same day, the House passed a continuing resolution that included $5 billion for the wall and $8 billion in disaster aid. This bill failed in the Senate. Trump's changing position caused consternation among Senate Republicans.
The shutdown started December 22 and Trump announced that he would cancel his planned trip to Mar-a-Lago for Christmas and stay in Washington, D.C. The meaning of the term "wall" was expected to be an aspect of the negotiations.