Counselor to the president Kellyanne Conway cast the speech as an opportunity for the president to speak to the nation, stressing that it will be "in his voice, in his words".
Vice President Mike Pence, in a round of television interviews on Tuesday morning, said Trump would use his address to explain to the American people that there is "a humanitarian and security crisis" at the southern US border.
He said he and Homeland Security Secretary Kirstjen Nielsen would brief lawmakers at the Capitol on Tuesday and Wednesday. This was clear at the beginning of the shutdown, because he had initially agreed to fund the government without money for a border wall - and he only called off that deal at the last minute when he saw how angry it made conservative media figures.
"We have an absolute crisis, and of criminals and gang members coming through".
Pence says he's seen "clips of previous presidents talking about the importance of border security and the importance of addressing illegal immigration".
U.S. Representative Veronica Escobar, a Democrat from El Paso, said the area where Gomez and his father turned themselves over to Border Patrol is on American soil and already fenced.
The president will continue pressing his case with a trip to the USA southern border on Thursday.
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When is the Trump prime-time address tonight?
Democrats have said they support increased border security measures such as additional USA border agents and technology, but have rejected the administration's claims about the security risks at the border and have raised concerns that Trump will use his speech to present a false narrative.
"This might be all about putting up a fight for the sake of the people who are watching, which are his base and his conservative allies out in conservative media", said CNN's White House correspondent Abby Philip.
Vice President Mike Pence said Trump had not made a decision about his threat to declare a national emergency and build a wall without congressional approval.
That's the same topic Trump says he plans to talk about Tuesday evening, and a subject on which he continues to lie repeatedly. John Cornyn of Texas, on Monday said an emergency declaration would add "new elements to this - court hearings and litigation that may carry this on for weeks and months and years", adding: "To me, injecting a new element in this just makes it more complicated".
Presidents have been using Oval Office addresses to make big announcements for decades.
It seems especially unlikely that Congress would terminate the potential border-related national emergency, given its current makeup, as doing so would require a joint resolution and a signature from the president.
"As we speak, we are in the middle of an unprecedented crisis on our middle of an unprecedented crisis on our middle of an unprecedented crisis on our southern border", he said October 30, 2016, speech in Albuquerque, New Mexico.
Pence told NBC's "Today" show on Tuesday, "I know the president has said that was his impression from previous presidents".