US House Speaker Nancy Pelosi has said that by invoking a national emergency to build a border wall, Donald Trump would be setting a precedent that would allow a future Democratic president to do the same for gun control.
The Kentucky Republican said he'd spoken personally to the president to get the assurance about signing the bill.
Funding is due to expire for a range of federal agencies on Friday.
The bill allocating just $1.3 billion for a barrier along the US-Mexico border - a far cry from the $5.7 billion requested by Trump - has been approved by the Senate and is now going through the House.
The legislation includes 1.37 billion USA dollars in new money to help build 55 miles (88.5 km) of new physical border barriers.
"Remember, Donald Trump signed that omnibus previous year, on promise he'd get his wall funding", she wrote.
Building the wall was a key election promise by Mr Trump. She also said that she may file a legal challenge if Trump went through with the national emergency.
The money, which would be taken from construction projects such as hospitals, family housing, and maintenance, would not be replenished until Congress passes another defense appropriations bill, the aides said.
Declaring a national emergency gives Trump the authority to move money from elsewhere in the federal budget and use it for a wall.More news: Real Madrid's Thibaut Courtois claims VAR right to rule out Ajax goal
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The top Democrat in Congress immediately denounced the president's move. "It's a humanitarian challenge to us", Ms Pelosi said.
With numerous Democrats' liberal base voters adamantly against Trump's aggressive attempts to curb immigration, four declared presidential hopefuls opposed the bill in the Senate: Cory Booker of New Jersey, New York's Kirsten Gillibrand, Elizabeth Warren of MA and Kamala Harris of California.
Congressional Republicans have been pointing Trump to pots of federal money he could tap for a wall to steer him away from using a politically hard emergency declaration to bypass Congress, a strategy sure to be challenged in court.
"He couldn't convince Mexico, the American people or their elected representatives to pay for his ineffective and expensive wall, so now he's trying an end-run around Congress in a desperate attempt to put taxpayers on the hook for it".
"If Trump oversteps his authority and abandons negotiations with Congress by declaring a fabricated national emergency, we won't only call his bluff, we will do what we must to hold him accountable", Becerra said in a statement. He also has a bill that would prevent Trump from raiding disaster relief funds. In the meantime, the White House has confirmed it is considering other options when it comes to border wall funding. The president is still threatening to declare a state of emergency in order to secure the necessary funds.
"Want to talk about a national emergency?"
Other Republicans said they were hoping for assurances Mr Trump will sign the bill if it gets to his desk, possibly later on on Thursday.
The almost 1,200 page legislation would fund nine Cabinet departments and dozens of other agencies through September 30.
"You can use military construction funding under a national emergency, but it actually stipulates that that has to be for a military goal and it would be hard to justify this, I think, is a military objective", says military budget expert Todd Harrison of the Center for Strategic and International Studies.