President Donald Trump speaks as he meets with NATO Secretary General Jens Stoltenberg in the Oval Office of the White House, Tuesday, April 2, 2019, in Washington.

President Donald Trump on Tuesday reiterated his threat to shut down the U.S.-Mexico border, insisting that any economic damage from the move would be overshadowed by the benefits of greater security.

Asked at the White House whether shutting the border would hurt the U.S. economy, Trump replied, "Sure, it will have a negative effect on the economy," noting that Mexico is "a very big trading partner."

"Trading is very important. The borders are very important. But security is what is the most important," Trump said at a joint appearance with NATO Secretary-General Jens Stoltenberg.

"All you hear me talking about is trade, but let me just give you a little secret," Trump continued. "Security is more important to me than trade. So we're going to have a slower border or a closed border."

Trump's remarks came as the top Republican in Congress, the nation's most influential business lobby, and even Trump's own top economic advisor all cautioned that the impact on American businesses of closing the massive U.S.-Mexico border could be disastrous.

"Closing down the border would have potentially catastrophic economic impact on our country," Senate Majority Leader Mitch McConnell, R-Ky., told reporters Tuesday. "I would hope we would not be doing that sort of thing."

Neil Bradley, chief policy officer at the U.S. Chamber of Commerce, went a step further, warning in a statement Monday that "even threatening to close the border to legitimate commerce and travel creates a degree of economic uncertainty that risks compromising the very gains in growth and productivity that policies of the Trump Administration have helped achieve."