Nancy Pelosi has said that the House will vote in the coming days on a resolution rejecting Donald Trump's national emergency declaration, while encouraging fellow Democrats to support the effort as they try to stop the president's push to expand efforts to build a barrier along the US-Mexico border.

In a letter, the California Democrat said Mr Trump's declaration "undermines the separation of powers and Congress' power of the purse, a power exclusively reserved by the text of the Constitution to the first branch of government, the Legislative branch, a branch co-equal to the Executive."

By invoking a national emergency, Mr Trump is claiming authority to shift federal funds appropriated by Congress for other purposes to be spent instead on his border wall.

Ms Pelosi announced that the House would move "swiftly" to pass a disapproval resolution written by Texan Democrat Joaquin Castro, although she did not specify an exact date and indicated that it would move through a House committee before coming to the floor.

"All Members take an oath of office to support and defend the Constitution," Ms Pelosi wrote. "The President's decision to go outside the bounds of the law to try to get what he failed to achieve in the constitutional legislative process violates the Constitution and must be terminated."

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Ms Pelosi's announcement formalises a strategy House Democrats settled on several days ago.

Democratic leaders had been anticipating Mr Trump's emergency declaration for weeks and had been working quietly behind the scenes on a two-pronged approach that would include passing a disapproval resolution to put Republicans on record on the matter - then eventually suing or joining with a lawsuit challenging the declaration in court.

House committee chairmen, including Judiciary chief and New York Democrat Jerry Nadler, also anticipate holding hearings on Mr Trump's decision.

The resolution is expected to sail through the Democratic-controlled House. Its fate in the Senate, where Republicans hold a 53-to-47 advantage, is less certain.

Maine's Republican senator Susan Collins became the first member of her party to say publicly that she would vote to reject Mr Trump's emergency declaration. At an event in her home state, she said Mr Trump's move is of "dubious constitutionality" and pledged to support a "clean" resolution opposing it.

Mr Castro's resolution is one page long, exercising authority reserved by Congress under the 1976 National Emergencies Act. Under that law, a disapproval measure filed in either congressional chamber is entitled to expedited consideration.

The House, with a sizeable Democratic majority and Ms Pelosi's announcement of support, is all but certain to pass the measure. While the resolution would need only a simple majority to pass the Senate, it is unclear how many Republican senators would actually rebuke Mr Trump by supporting it.

Indeed, some of the party's critics who said it would be a bad idea for Mr Trump to get the funding without Congress have silenced their pushback - or even reversed course entirely - after the president's decision to ignore their advice.

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Should the measure pass the Senate, Trump officials have already told surrogates he will veto the bill. Congress is unlikely to have the numbers to override that veto.

The bigger threat to Mr Trump's declaration is the courts. A group of states led by Democratic governors have filed suit against the administration, while other groups - representing border landowners and other potential parties who could be affected by the border wall or the shifting of appropriated funds to pay for it - are either preparing lawsuits or have filed them already.

Democratic lawmakers are expected to aggressively defend Congress' constitutional power to appropriate money in the courts, arguing that Mr Trump cannot unilaterally shift other resources to a project that lawmakers have otherwise refused to fund.

But Trump allies argue that Congress itself placed few bounds on presidential emergency powers and that courts should not intervene if the congressional disapproval process fails.

House lawyers have not determined whether the chamber will join or support one of those lawsuits or instead file suit against Mr Trump separately, Democratic aides familiar with the deliberations said last week.