WASHINGTON — With no breakthrough in sight, US President Donald Trump will argue his case to the nation Tuesday night that a "crisis" at the US-Mexico border requires the long and invulnerable wall he's demanding before ending the partial government shutdown. Hundreds of thousands of federal workers face missed paychecks Friday as the shutdown drags through a third week.

Trump's Oval Office speech — his first as president — will be followed by his visit Thursday to the southern border to highlight his demand for a barrier. White House press secretary Sarah Huckabee Sanders tweeted that he will use the visit to "meet with those on the front lines of the national security and humanitarian crisis."

The administration is also at least talking about the idea of declaring a national emergency to allow Trump to move forward on the wall without Congress approving the $5.6 billion he wants.

Vice President Mike Pence said the White House counsel's office is looking at the idea. Such a move would certainly draw legal challenges, and Trump — who told lawmakers he would be willing to keep the government closed for months or even years — has said he would like to continue negotiations for now.

Trump's prime-time address will be carried live by ABC, CBS, CNN, Fox Broadcasting, Fox News Channel, Fox Business Network, MSNBC and NBC.

House Speaker Nancy Pelosi and top Senate Democrat Chuck Schumer called on the networks to give Democrats a chance to respond.

"Now that the television networks have decided to air the President's address, which if his past statements are any indication will be full of malice and misinformation, Democrats must immediately be given equal airtime," they wrote in a joint statement released Monday night.

As Trump's speech and border visit were announced, newly empowered House Democrats — and at least a few Republican senators — stepped up pressure on GOP lawmakers to reopen the government without giving in to the president's demands The closure, which has lasted 17 days, is already the second-longest in history and would become the longest this weekend.

Leaning on Senate Republicans, some of whom are growing anxious about the impact of the shutdown, Pelosi said the House would begin passing individual bills this week that would reopen federal agencies, starting with the Treasury Department to ensure Americans receive their tax refunds .

The White House moved to pre-empt the Democrats, telling reporters Monday that tax refunds would be paid despite the shutdown. That shutdown exemption would break from the practice of earlier administrations and could be challenged.

"There is an indefinite appropriation to pay tax refunds. As a result ... the refunds will go out as normal," said Russell Vought, acting director of the White House budget office.

There were other signs that administration was working to control the damage from the shutdown, which has furloughed 380,000 federal workers and forced another 420,000 to work without pay. The National Park Service said it was dipping into entrance fees to pay for staffing at some highly visited parks to maintain restrooms, clean up trash and patrol the grounds, after reports of human waste and garbage overflowing in some spots.

Over the weekend, the federal agency tasked with guaranteeing US airport security acknowledged an increase in the number of its employees missing work or calling in sick.

But Trump and the Transportation Security Administration pushed back on any suggestion that the call-outs at the agency represented a "sickout" that was having a significant effect on US air travel. TSA said it screened more than 2.2 million passengers Sunday, a historically busy day due to holiday travel. Ninety percent waited less than 15 minutes, the agency said.

"We are grateful to the more than 51,000 agents across the country who remain focused on the mission and are respectful to the traveling public," said TSA spokesman Michael Bilello.

The talks over ending the shutdown have been at an impasse over Trump's demand for the wall. He has offered to build the barrier with steel rather than concrete, billing that as a concession to Democrats' objections. They "don't like concrete, so we'll give them steel," he said.

But Democrats have made clear that they object to the wall itself, not how it's constructed. They see it as immoral and ineffective and prefer other types of border security funded at already agreed-upon levels.

"Maybe he thinks he can bully us. But I'm from Brooklyn. You let a bully succeed, you'll be bullied again worse," Schumer said at a breakfast with the Association for a Better New York.

At the White House, spokeswoman Mercedes Schlapp complained that Democratic leaders have yet to define what they mean when they say they are for enhancing border security.

"Democrats want to secure the border? Great. Come to the table," she said Monday. "We are willing to come to a deal to reopen the government."