They say he is using false claims and manufacturing a crisis to carry out his 2016 presidential campaign promise for a wall that he said at the time would be paid for by Mexico.
Global News will be streaming Trump's Oval Office address live at 9 p.m. ET on our website and on YouTube. Hundreds of thousands of federal workers are going without pay, and government disruptions are hitting home with everyday Americans.
There were two audiences for Donald Trump's address to the nation on Tuesday night.
The dispute over wall funding - Trump is demanding $5.7 billion USA to help build it - led to a stalemate in Congress over funding for parts of the government. But Trump rejects that legislation because it doesn't have funding for his border wall.
Sen John Thune of South Dakota, the GOP whip, said he doesn't think the emergency declaration is the right move and "I prefer that we get this resolved the old-fashioned way".
Rep. Steny Hoyer of Maryland, the House Democrats' majority leader, said Tuesday morning that he believes Trump lacks the legal authority to declare a national emergency as a mechanism for building his border wall by sidestepping the congressional purse-strings.
Instead, he will try to make the public case for his $5.6 billion border wall, a demand that is the heart of a standoff that has led to a partial government shutdown.
The government shutdown entered day 17 on 8 January after a series of discussions between the Trump administration officials and Democrats failed to yield an agreement on funding for the wall.
Len Saunders, an immigration lawyer based near the border in Blaine, Wash., said those border guards - many of whom are supportive of the president's border-security efforts - may soon stop showing up for work as they begin to feel the shutdown's financial impact.More news: New Captain Marvel Trailer Has More SHIELD And Shape-Shifting Skrulls
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But with many Americans far from sold on Trump's lurid claims about illegal immigrants, criminals and terrorists overwhelming the border, the speech faces its own high barrier: credibility.
Tonight - and throughout this debate and his presidency - President Trump has appealed to fear, not facts.
Trump, who has long railed against illegal immigration at the border, has recently seized on humanitarian concerns to argue there is a broader crisis that can only be solved with a wall along the US-Mexico border.
House Speaker Nancy Pelosi (D-Calif.) and Senate Minority Leader Chuck Schumer (D-N.Y.) responded to the planned speech by asserting Democrats should have airtime to respond to Trump.
Though both Democrats and Republicans agree there is a crisis at the border, critics have accused Mr Trump of greatly exaggerating the problem.
America would be "safer than every before" under the proposal, which includes "cutting-edge technology for detecting drugs, weapons, illegal contraband and many other things", he said.
It invited representatives from cable news outlets to a White House lunch with Trump ahead of the address, a gesture that is typically reserved for the lead-up to State of the Union speeches.
The number of illegal border crossings is down from 1.6 million in 2000 to less than 400,000 a year ago.
A new opinion poll suggests just over half of Americans (51%) blame President Trump for the government shutdown.