WASHINGTON — President Trump lashed out at Congress on Thursday for failing to deliver his long-promised border wall, unleashing a tweet that accused Democrats of “obstructing” border security and demanded that “REPUBLICANS MUST FINALLY GET TOUGH!”

The trouble for Trump is that it’s his own Republican allies – not just his political opponents – who have been standing in the way.

The same Republican lawmakers who rushed through the tax bill Trump wanted, confirmed his first Supreme Court pick and are fighting to defend his second, and have remained largely deferential amid multiple scandals, have taken a far different approach when it comes to one of Trump’s most memorable campaign promises – deeming the wall to be impractical, unrealistic and too costly.

“People can climb over the wall or go under the wall or through the wall. We’ve seen that in different places,” said Sen. John Cornyn, Texas, the No. 2 Republican in the Senate, explaining why a system of technology, infrastructure and personnel is preferable to a physical wall. “If it’s just unattended without sensors, without technology, without people, then it won’t work.”

Another powerful Republican, Senate Appropriations Committee Chairman Sen. Richard Shelby, Ala., said he told Trump that funding a 2,000-mile-long wall could jeopardize money for the military and other core programs.

“Some things are reachable and some things aren’t,” Shelby said he told Trump. “I’m committed to securing the borders, whatever it takes in this country; it’s something we haven’t done. But I’m also committed to funding the government.”

The Republican recalcitrance on the wall underscores the extent to which immigration and border issues continue to roil the party in the two years since Trump swept into office vowing to take a hard-line approach on the issue.

The idea of building a wall along the U.S.-Mexico border remains hugely popular among Trump’s core supporters, with chants of “build that wall” still ringing out at his rallies, and numerous candidates in this year’s midterms echoing Trump’s rhetoric.

But the issue is not as clear-cut for many other Republicans. Border state lawmakers face concerns from landowners and businesses that could face disruption by the construction.