Trump warns fellow Republicans as Congress prepares to vote on rejecting his national emergency

President Donald Trump on Monday 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 U.S.-Mexico border.

A measure is expected to pass easily in the Democratic-controlled House on Tuesday, 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 criticized 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 Monday. "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 authorized 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 a rejection of his Trump's emergency declaration and send it to his desk.

On Friday, Trump said 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 fresh backlash from fellow Republicans.

A group of 23 former Republican members of Congress have written a 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 honor your oath of office and to protect the Constitution and the responsibilities it vested in Congress," says the letter.

Its signatories include former senators John Danforth, Mo., Chuck Hagel, Neb., Gordon Humphrey, N.H., Richard Lugar, Ind., Olympia Snowe, Maine, and 18 former House members.

A bipartisan group of 58 former senior national security officials also plans to issue a statement Monday saying that "there is no factual basis" for Trump's proclamation of a national emergency on the U.S.-Mexico border.

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.

The Washington Post’s Mike DeBonis and By Ellen Nakashima contributed to this report.