"The President's sham emergency declaration and unlawful transfers of funds have undermined our democracy, contravening the vote of the bipartisan Congress, the will of the American people and the letter of the Constitution," Pelosi said in a statement, cited by CNN.
The lawsuit, which has not been filed yet, will argue that Trump's decision to transfer funds from appropriated accounts to a southern border wall violates the Appropriations Clause of the Constitution. The clause — found in Article I, Section 8 — gives Congress power over the designation of federal spending.
“Congress […] must reassert its exclusive responsibilities reserved by the text of the Constitution and protect our system of checks and balances,” Pelosi added.
The national emergency declaration in response to the crisis on the southern border came after a 35-day government shutdown over border security. Previously, Trump declared a national emergency to tackle illegal migration in February after demanding $8 billion in federal funding from Congress for border security, including the construction of a physical barrier along the southern US border. The US president stressed the necessity of the measure, saying the wall is needed to prevent illegal immigrants from pouring into the US. Following Trump's declaration, the House and the Senate passed resolutions to overturn the national emergency, arguing that the President had unconstitutionally bypassed Congress, yet didn’t have sufficient votes to override the President's veto.