President Trump on Tuesday threatened to shut down the government, if necessary, to make sure his proposed wall at the Mexican border is approved and funded by lawmakers.

"Build that wall. Now the obstructionist Democrats would like us not to do it, but believe me if we have to close down our government, we're building that wall," Trump said Tuesday during a rally in Phoenix.

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Funding for Trump's proposed border wall, a promise that was at the center of his 2016 presidential bid, is poised to be the center of debate this fall as Congress negotiates a new government funding bill.

If a new bill is not passed, the government will shut down on Oct. 1.

Democrats, however, are staunchly opposed to funding for the wall, setting up a challenge and tense negotiations over the government's funding.

The Department of Homeland Security accepted designs for the wall earlier this year. Vendors were instructed that the wall should be at least 18 feet high, have features to prevent climbing over and tunneling beneath the structure, and to make the designs "aesthetically pleasing" on the U.S. side.