Trump warns Republicans to stay in line over border vote

US President Donald Trump is on his way to Vietnam to meet North Korean leader Kim Jong Un. Photo / AP

US President Donald Trump cautioned fellow Republicans against falling into a Democratic "trap" as lawmakers prepare to vote this week on legislation rejecting his declaration of a national emergency at the US-Mexico border.

A measure is expected to pass easily in the Democratic-controlled House tomorrow, forcing a difficult vote for GOP senators, who will have to weigh whether to support Trump on a move that even some in his party have criticised as circumventing the will of Congress.

"I hope our great Republican Senators don't get led down the path of weak and ineffective Border Security," Trump said in a tweet today. "Without strong Borders, we don't have a Country - and the voters are on board with us. Be strong and smart, don't fall into the Democrats 'trap' of Open Borders and Crime!"

Trump is attempting to use the emergency declaration to justify spending significantly more on barriers at the border than what Congress authorised in a compromise intended to avert another partial government shutdown.

If Senate Democrats are united, they will need only four Republican defections to pass the rejection of Trump's emergency declaration and send it to his desk.

Trump said last week that he would veto the measure "100 percent" if that happened. And he predicted that Congress would be unable to muster the votes to override his veto.

As the House vote approaches, Trump is facing a fresh backlash from fellow Republicans.

A group of 25 former Republican members of Congress has written an open letter urging a termination of the emergency declaration.

The letter argues that Trump is encroaching on Congress's "power of the purse" and urges current lawmakers to stand up for its constitutional powers.

"We who have served where you serve now call on you to honour your oath of office and to protect the Constitution and the responsibilities it vested in Congress," says the letter.

Its signers include former senators John Danforth of Missouri, Chuck Hagel of Nebraska, Gordon Humphrey of New Hampshire, Richard Lugar of Indiana, Olympia Snowe of Maine, and 18 former House members.

A bipartisan group of 58 former senior national security officials also plans to issue a statement saying that "there is no factual basis" for Trump's proclamation.

The former officials' statement, which will be entered into the Congressional Record, is intended to support lawsuits and other actions challenging the national emergency proclamation and to force the Administration to set forth the legal and factual basis for it.