President Trump's plans to declare a national emergency in order to fund his long-promised border wall will enable his administration to move $8 billion from various federal agencies to fund the project, a senior administration official told Fox News late Thursday.

The news comes as Trump is expected to sign a House border security package that provides $1.4 billion for the project, which is far below the $5.7 billion Trump insisted he needed and would finance just a quarter of the 200-plus miles he wanted to be defended against illegal immigrants.

Sarah Sanders, the White House press secretary, said earlier Thursday that Trump will sign the spending bill and declare a national emergency at the same time Friday morning.

The White House said Trump would sign the legislation but act unilaterally to get more funding, prompting condemnations from Democrats and threats of lawsuits from states and others who might lose federal money or said Trump was abusing his authority.

In an unusual joint statement, House Speaker Nancy Pelosi, D-Calif., and Senate Minority Leader Chuck Schumer, D-N.Y., said such a declaration would be “a lawless act, a gross abuse of the power of the presidency and a desperate attempt to distract” from Trump’s failure to force Mexico to pay for the wall, as he’s promised for years.

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“Congress will defend our constitutional authorities,” they said. They declined to say whether that meant lawsuits or votes on resolutions to prevent Trump from unilaterally shifting money to wall-building, with aides saying they’d wait to see what he does.

Fox News' Gregg Re, Edmund DeMarche, Ellison Barber and the Associated Press contributed to this report