They plan to say there is "no factual basis" for Trump's proclamation.
"Under no plausible assessment of the evidence is there a national emergency today that entitles the president to tap into funds appropriated for other purposes to build a wall at the southern border," the statement will say, according to The Post.
Signatories of the letter include Madeleine Albright, former defense secretary under President Bill Clinton; Chuck Hagel, former defense secretary under President Barack Obama; Eliot Cohen, a State Department counselor under President George W. Bush; John Kerry, the second secretary of state who served under Obama; Susan Rice, Obamas national security adviser; and Leon Panetta, who was CIA director and defense secretary under Obama.
They point out in their letter that illegal border crossings are at near 40-year lows, there is no terrorism or violent crime crisis at the border, and that a wall is unlikely to stop drug flows because "the overwhelming majority of opioids" come in through legal ports of entry, according to The Post.
On Tuesday, the House will vote on a resolution put forth by Democratic Rep. Joaquin Castro of Texas that would terminate the national emergency declaration.
Trump's emergency declaration ventured into new territory to circumvent Congress and redirect military funding for other purposes to the border wall.
House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer described the declaration as unlawful.
Joe Perticone contributed to this report.
