[UPDATE: Trump makes grisly case for his border wall, while Democratic leaders accuse him of fear-mongering]

House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer were set to push back on President Donald Trump's Oval Office address Tuesday night calling for the construction of a U.S.-Mexico border wall.

Pelosi and Schumer have taken a firm stance against including any money for a southern border barrier as part of a deal to reopen nine federal agencies that have been affected by a partial government shutdown.

Trump, however, has refused to accept any funding agreement that excludes billions of dollars toward the construction of a border wall, calling the situation at the border a "Humanitarian and National Security crisis."

By Tuesday, the shutdown of about 25 percent of the government had stretched out to 18 days, making it the second-longest on record.

Trump had initially said he would be "proud" to shut down the government in pursuit of a wall, which he has described as a necessity for protecting the very existence of the country. But he has since shifted to blame the gridlock on Democrats, many of whom have in turn labeled the wall a "vanity project" for the president.

Trump tweet

The two Democratic leaders were expected to deliver their response to Trump from the speaker's balcony in the U.S. Capitol building after the president's televised 9 p.m. ET address had concluded.

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