US President Donald Trump is set to argue that an immigration "crisis" requires his long-promised wall along the Mexican border.
Mr Trump will make his case in his first prime-time speech from the Oval Office at 9pm EST (3pm NZT), ahead of a trip to the border on Thursday.
A partial government shutdown has been in effect for 17 days after lawmakers failed to break a budget impasse.
The president is insisting that $US5 billion ($NZ7.4b) be included for the border wall.
White House press secretary Sarah Huckabee Sanders said in a tweet that Mr Trump would use his visit to the border on Thursday to "meet with those on the front lines of the national security and humanitarian crisis".
Ahead of the speech on Tuesday, Vice-President Mike Pence said Mr Trump's decision to address the nation "comes from this president's deep desire to do his job to protect the American people".
Amid his talk of crisis, Mr Trump is considering declaring the border situation a national emergency, which could get him out of an impasse by enabling him to bypass Congress' mandate to approve federal spending and to build the wall without its approval. However, such a step would likely face an immediate legal challenge.
Mr Trump has long maintained that a border wall is needed to stem the flow of illegal immigration and drugs, and in recent weeks has made the issue a priority. Democrats, who now control the House of Representatives, have consistently opposed it, calling it an expensive, inefficient and immoral way of trying to resolve immigration issues.
All major US television networks agreed to broadcast Mr Trump's speech, prompting Democrats to seek equal air time. House Speaker Nancy Pelosi and Senate Democratic Leader Chuck Schumer will deliver a televised response after Mr Trump speaks on Tuesday night.
Democrats have said they support increased border security measures such as additional US border agents and technology, but have rejected the administration's claims about the security risks at the border and have raised concerns that Mr Trump will use his speech to present a false narrative.
The decision by the major US TV networks to set aside at least eight minutes of airtime for the speech has sparked controversy and fuelled debate on social media.
Critics pointed out that the networks aired President George W Bush's prime-time address on immigration in 2006, but did not air one by President Barack Obama in 2014.
Some reporters have called on the White House to provide the text of his remarks in advance to be fact-checked.
On Monday the majority of the living US presidents disputed Mr Trump's claim that some of them have privately told him they regret not building a border wall during their administrations.
"I have not discussed the border wall with President Trump, and do not support him on the issue," former President Jimmy Carter said in a statement issued by the Carter Center on Monday.
A spokesperson for George W Bush said that he never discussed the border wall with Mr Trump. A representative for Bill Clinton denied it and said that the two men have not spoken since Mr Trump took office, according to CBS.
Barack Obama has not explicitly denied Mr Trump's claim, but has extensively campaigned against the president on the issue.
Federal employees will feel the pinch from the shutdown on Friday, when they will miss their paychecks for the first time, unless a deal is reached. The shutdown, which has left some 800,000 government workers furloughed or working without pay, is also affecting national parks, airline security screening, housing and food aid, and economic data.
"This isn't about Democrats not wanting to talk about border security," Democratic US Senator Chris Murphy told MSNBC. "It's about making sure that the federal workforce isn't used over and over again as a hostage for the president's campaign promises."
- Reuters and BBC