President Trump has threatened to shut down the government if Congress fails to fund the U.S.-Mexico border wall.

“Believe me, if we have to close down our government, we’re building that wall,” he said at a rally in Phoenix, Arizona, Aug. 22.

Trump’s comments came when lawmakers are trying to pass legislation to fund the government beyond the 2017 fiscal year, which ends on Sept. 30. During his campaign, Trump promised to build a wall along the U.S.-Mexico border and have Mexico pay for it.

Many lawmakers, including Senate Minority Leader Chuck Schumer, have rejected the idea outright.

“If the President pursues this path, against the wishes of both Republicans and Democrats, as well as the majority of the American people, he will be heading towards a government shutdown which nobody will like and which won’t accomplish anything," Schumer said in a statement Aug. 23. “In April, Democrats and Republicans came together to pass a strong, bipartisan appropriations bill that increased funding for our troops, scientific research, border security, and more."

Building the wall has been a priority for Trump. Earlier this year when the spending package for border wall construction was stripped from the fiscal 2017 funding bill, Trump signed the bill but vowed to try again.

“Our country needs a good ‘shutdown’ in September to fix mess!” Trump tweeted at the time.

The 2013 government shutdown cost the economy $24 billion. About 850,000 federal employees were furloughed. It cost the government $2 billion in lost productivity.

Having to live with constant threats of a government shutdown every year is extremely demoralizing to the federal workforce and damaging the government’s ability to recruit and retain the best and the brightest.