UPDATE 1-Trump set to declare border emergency, sign shutdown-averting bill

(Adds timing of Trump's remarks in paragraph 3, time of his speech)

WASHINGTON, Feb 15 (Reuters) - U.S. President Donald Trump was poised on Friday to declare a national emergency at the U.S.-Mexico border, a move that Democrats vowed to challenge as an unconstitutional attempt to fund his proposed border wall without approval from Congress.

Trump was also expected to sign a bipartisan government spending bill approved by Congress on Thursday that would prevent another federal shutdown by funding several agencies that otherwise would have closed on Saturday morning.

The Republican president was scheduled to deliver remarks on the issue at 10 a.m. EST (1500 GMT) in the Rose Garden at the White House.

The bill, lacking any money for his wall, is a defeat for Trump in Congress, where his demand for $5.7 billion in barrier funding yielded no results, other than a record-long 35-day December-January partial government shutdown that damaged the U.S. economy and his poll numbers.

Reorienting his wall-funding quest toward a legally uncertain strategy based on declaring a national emergency could plunge Trump into a lengthy battle with Democrats and divide his fellow Republicans.

Even before the White House said on Thursday that Trump would declare an emergency, Republican senators, while sympathetic to his view that the southern border is in crisis, were skeptical of the declaration that would shift funds to the wall from other commitments set by Congress.

"No crisis justifies violating the Constitution," Republican Senator Marco Rubio said on Twitter on Thursday.

Republican Senator John Cornyn told reporters on Capitol Hill he had concerns about an emergency declaration. He said it "would not be a practical solution, because there would be a lawsuit filed immediately and the money would be presumably balled up ..."

Some Republicans were more supportive of Trump's tactic. "Im not uncomfortable. I think the presidents probably on pretty solid ground," said Republican Senator Richard Shelby.

Fifteen Democrats in the Republican-controlled Senate introduced legislation to prevent the transfer of funds from accounts Trump likely would target to pay for his wall.

A senior White House official said the administration had found nearly $7 billion to reallocate to the wall, including $600 million from a Treasury Department forfeiture fund, $2.5 billion from a Defense Department drug interdiction fund and $3.5 billion from a military construction budget.

The funds would cover just part of the estimated $23 billion cost of the wall promised by Trump along the 2,000-mile (3,200-km) border with Mexico.

The Senate Democrats' bill also would stop Trump from using appropriated money to acquire lands to build the wall unless specifically authorized by Congress.

'PHONY NATIONAL EMERGENCY'

Trump says the wall is needed to curb illegal immigrants and illicit drugs streaming across the southern border despite statistics that show illegal immigration there is at a 20-year low and that many drug shipments are likely smuggled through legal ports of entry.

Democratic Representative David Price urged lawmakers on the House floor to block Trump's "phony national emergency."

Representative Jerrold Nadler, chairman of the Judiciary Committee in the Democratic-controlled House of Representatives, said he would back a joint resolution to terminate the president's emergency declaration under the National Emergencies Act, and pursue "all other available legal options."

On Thursday evening, the Senate passed the government funding bill by a vote of 83-16, and the House by 300-128, with 86 House Republicans voting in favor.

Trump was expected to sign it and declare an emergency, then fly to his private golf club in Florida for a holiday weekend break.

(Reporting by Richard Cowan and David Morgan; Additional reporting by Steve Holland, Susan Cornwell, Makini Brice and Eric Beech; Writing by Kevin Drawbaugh and Peter Cooney; Editing by Jeffrey Benkoe)