Until then, here is some important background.
When is it a “crisis”?
Saying a situation is a crisis does not make it so, but Mr. Trump and other senior administration officials have been using the word repeatedly in recent days to describe the state of affairs along the border.
They point to a direct connection between the flow of drugs from Mexico and the opioid epidemic in the United States. They say that migrants, particularly women and children, are victims of crime as they travel to the United States. And Mr. Trump has also strongly suggested that terrorists may be slipping across the border.
These threats are greatly exaggerated, if not fabricated. Migrant border crossings have been declining for nearly two decades. The majority of heroin enters the United States through legal ports of entry, not through open areas of the border. Immigrants are less likely to commit crimes in the United States than native-born Americans. The State Department said in a recent report that there is “no credible evidence” that terrorist groups had sent operatives to enter the United States through Mexico.
Nancy Pelosi and Chuck Schumer will deliver the Democratic response.
Speaker Nancy Pelosi of California and Senator Chuck Schumer of New York, the minority leader, will speak from the Capitol after Mr. Trump finishes his remarks from the Oval Office. The dueling speeches from opposite sides of Pennsylvania Avenue will unfold at the close of Day 18 of a government shutdown over Mr. Trump’s demand for $5.7 billion for a border wall that Democrats have steadfastly opposed.
The Democratic leaders’ decision to select themselves as the message-bearers to counter the president underscores how a partisan power struggle in a new era of divided government is undergirding the discussions over resolving the shutdown, even as the paychecks of hundreds of thousands of federal workers and benefits for millions of Americans hang in the balance.
Senator Bernie Sanders, independent of Vermont, who ran unsuccessfully for the Democratic presidential nomination in 2016, also plans to make a public response to Mr. Trump, which his office said would be streamed live on social media platforms after Ms. Pelosi and Mr. Schumer conclude their remarks. The move was reminiscent of how a fractious Republican Party responded to State of the Union addresses when Barack Obama was president: Republican leaders designated a formal response and Tea Party-aligned conservatives chose their own messengers to deliver a different rebuttal.
— Julie Hirschfeld Davis
What about a national emergency?
Mr. Trump could also try to circumvent Congress by declaring a national emergency to order that the wall be built, but administration officials who had seen a draft copy of his speech said the president was not preparing to do so.
The rationale for the emergency power is to give the president the capacity to act quickly to deal with matters like an urgent security threat. For example, President George W. Bush invoked emergency powers after the Sept. 11, 2001, attacks.
— Michael Tackett
Senate Democrats have taken the floor in protest.
A dozen Senate Democrats are taking the floor this evening for a talkathon-style protest calling on Mr. Trump and Senate Republicans to end the government shutdown. Led by Senators Tim Kaine of Virginia and Jeanne Shaheen of New Hampshire, it is yet another effort to raise the pressure on Senator Mitch McConnell, Republican of Kentucky and the majority leader, to allow a vote on legislation that would reopen the government.
Earlier Tuesday, Senate Democrats voted against advancing a package of bipartisan Middle East policy bills slated for consideration this week to further press Republicans. Mr. McConnell said on the Senate floor on Tuesday that he would “not waste time on show votes.” But he is also being pushed by vulnerable members of his own party up for re-election in 2020 to intervene and end the shutdown.
— Catie Edmondson
The decision by major broadcast networks to carry Mr. Trump’s address live in prime time has set off a fierce debate.
What is normally an easy decision for network executives — granting airtime to a sitting president to address the nation — led to hours of hand-wringing by journalists and producers wary of giving a platform to a president whose public remarks, particularly on immigration, have been marked by untruths and misleading claims. Liberals wondered why news outlets would defer to a president who, hours earlier, had used Twitter to label journalists “the Enemy of the People,” “the real Opposition Party” and “crazed lunatics.”
Eventually, ABC, CBS, NBC and Fox agreed to a request by Mr. Trump for the airtime, forgoing their 9 p.m. entertainment shows — and millions of dollars in associated ad revenue — for his Oval Office appearance, in which he plans to address the government shutdown. The networks said on Tuesday that they would also broadcast the Democratic response.
— Michael M. Grynbaum