Watch live: Trump’s immigration address

President Donald Trump will make his case to Americans on Tuesday that a wall is urgently needed to resolve what he calls a crisis at the US-Mexico border, trying to win support in a dispute that has sparked an 18-day partial government shutdown.

Trump’s prime-time address, scheduled for 9 p.m. will be his latest attempt to persuade Congress to fund construction of a huge barrier along the 2,000-mile border. 

Democrats in control of the US House of Representatives oppose the plan and said they would not fund it.

Trump has in recent days backed away from demanding a concrete wall to stop illegal immigration, shifting instead to touting steel fences in the hope that will draw more support. But negotiations between the two sides have failed.

Related: What happens if Trump declares a state of emergency to get his border wall?

Trump is considering declaring the border situation a national emergency, which could allow him to bypass Congress’ mandate to approve federal spending and to build the border wall without its approval. 

Such a step, however, would likely face immediate legal challenges, and Democrats accuse the Republican president of manufacturing a crisis.

In an evening meeting on Capitol Hill between Vice President Mike Pence and congressional Republicans, the emergency declaration idea was not discussed, House Republican leader Kevin McCarthy told reporters. 

The White House has not said why the situation might constitute a national emergency. In television interviews on Tuesday morning, Pence said Trump would tell Americans there is “a humanitarian and security crisis” at the border.

Related: The ‘real’ border crisis: The US immigration system isn’t built for kids and families

A Reuters/Ipsos poll released on Tuesday found that 51 percent of adults mainly blamed Trump for the shutdown, up 4 percentage points from late December, while 32 percent blamed congressional Democrats and 7 percent faulted Republicans in Congress.

Republican lawmakers have increasingly expressed concerns about Trump’s handling of the long-running dispute over the border wall.

Mac Thornberry, the senior Republican on the House of Representatives Armed Services Committee, told reporters he opposed using military funds to build the wall, as Trump has suggested.

“Border security is very important,” Thornberry said, adding: “It is not a responsibility of the Department of Defense.”

On Monday, in an interview with the Argus Leader newspaper in South Dakota, Republican Senator Mike Rounds said the prolonged government shutdown was “frustrating,” citing worries about the impact on federal workers in his home state and the possibility that low-income residents could see interruptions in food aid.  

Agriculture Secretary Sonny Perdue said on Tuesday that his department was taking steps to ensure full benefits through February for the Supplemental Nutrition Assistance Program, popularly known as food stamps, for low-income Americans.  

The pro-business US Chamber of Commerce on Tuesday called on Congress and Trump to end the shutdown, saying it “is harming the American people, the business community, and the economy.” 

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 after winning congressional elections in November, call it an expensive, inefficient and immoral way of trying to resolve immigration issues. 

The dispute over wall funding – with Trump demanding $5.7 billion just for this year to help build it – led to a stalemate in Congress over funding for parts of the government. 

About a quarter of US agencies have been shut down since Dec. 22 and hundreds of thousands of government workers are likely to miss paychecks this week.

While Trump paints a picture of an “unprecedented crisis” of illegal immigration at the US-Mexican border, illegal crossings there have dropped dramatically in recent years. 

There were nearly 400,000 apprehensions on the border in the 2018 fiscal year, far lower than in the early 2000s when arrests regularly topped 1 million annually. [https://tmsnrt.rs/2H2MCdr]

But in recent years, the border has seen many more Central American families and unaccompanied children turning up to seek asylum, sometimes in caravans of thousands of people, and the government does not have the facilities to take care of them.

Despite the focus on the US-Mexico border, most immigrants living in the United States without authorization entered with visas and then stayed on when their documents expired.

All major US television networks agreed to broadcast 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 Trump speaks.

Democrats say they support increased border security measures such as additional US border agents and technology, but have rejected Trump’s assertions about the security risks.

House Majority Leader Steny Hoyer said if Trump declared a national emergency, he would be like authoritarian leaders of other nations who use martial law to circumvent the rule of law. 

“We don’t think that’s the American way,” Hoyer told reporters on Tuesday.   

Trump will continue pressing his case for the wall with a trip to the border on Thursday.     

The shutdown has left some 800,000 government workers furloughed or working without pay, and is also affecting national parks, airline security screening, housing and the release of economic data. 

Trump’s promise of a wall was a signature issue in his 2016 election campaign. He said Mexico would pay for the wall, although Mexico was always clear it would not, and he has now turned to Congress for the money.