WASHINGTON — With no breakthrough in sight to end the partial government shutdown, President Donald Trump plans a Tuesday night address to the nation and a Thursday visit the U.S.-Mexico border to highlight his demands for a border wall.

Trump said he would discuss the “Humanitarian and National Security crisis on our Southern Border” at 9 p.m. He maintains that more than $5 billion for a wall is necessary to secure the border.

As the shutdown lurched into a third week, many Republicans watched nervously from the sidelines as hundreds of thousands of federal workers went without pay and government disruptions hit the lives of ordinary Americans.

Trump has offered to build the barrier with steel rather than concrete, billing that as a concession to Democrats' objections to a solid wall. They “don't like concrete, so we'll give them steel,” he has said.

But the Democrats have made clear they see a wall as immoral and ineffective and prefer other types of border security funded at already agreed-upon levels.

White House officials affirmed Trump's funding request in a letter to Capitol Hill after a meeting Sunday with senior congressional aides led by Vice President Mike Pence at the White House complex yielded little progress. The letter from Office of Management and Budget Acting Director Russell Vought sought funding for a “steel barrier on the Southwest border.”

The White House said the letter, as well as details provided during the meeting, sought to answer Democrats' questions about the funding request. Democrats, though, said the administration still failed to provide a full budget of how it would spend the billions requested.

The administration letter includes a request for $800 million for “urgent humanitarian needs,” a reflection of the growing anxiety over migrants traveling to the border — which the White House said Democrats raised in the meetings. And it repeats some existing funding requests for detention beds and security officers, which have already been panned by Congress and would likely find resistance among House Democrats.

House Speaker Nancy Pelosi intends to begin passing individual bills to reopen agencies in the coming days, starting with the Treasury Department to ensure Americans receive their tax refunds.

Among Republicans expressing concern was Sen. Susan Collins, R-Maine, who said Senate Majority Leader Mitch McConnell should take up bills from the Democratic-led House.

“Let's get those reopened while the negotiations continue,” Collins said on NBC's “Meet the Press.”

Adding to concerns, federal workers might miss this week's paychecks.

Acting chief of staff Mick Mulvaney said on NBC's “Meet the Press” that if the shutdown continues into Tuesday, “then payroll will not go out as originally planned on Friday night.”

Over the weekend, the federal agency tasked with guaranteeing U.S. airport security acknowledged an increase in the number of its employees calling off work. But Trump and the Department of Homeland Security pushed back on any suggestion that the call-outs represented a “sickout” that was having a significant effect on U.S. air travel.

At the White House, on Monday, 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. “We are willing to come to a deal to reopen the government.”