WASHINGTON — Just when House Republicans needed Donald Trump’s backing the most – on their big immigration overhaul – he dashed off a presidential tweet on Friday saying they should quit wasting their time on it.

It’s hardly the first time the president has abandoned his allies in a moment of need. Over and over, Trump has proven himself a saboteur, willing to walk away from promises and blow up a deal, undermining the Republican agenda in Congress.

“You just fear that tweet in the morning,” said Rep. Ileana Ros-Lehtinen of Florida. The retiring Republican said members of Congress can’t help but think, “Oh no, how many policies will you undo by the day’s end? Because the day’s not over. Heck, it’s not even noon yet. How many times could he change his mind?”

On Capitol Hill on Friday, the mood was gloomy, particularly among the more centrist Republicans who have been pushing the party’s immigration compromise. That bill would provide $25 billion for Trump’s border wall and set new limits on family visas in favor of merit-based entry – but also create a path to citizenship for young “Dreamers.”

It seemed to be losing – rather than gaining – support ahead of rescheduled voting next week. Trump had publicly backed the bill earlier in the week.

“It’s a horrifically chilling signal,” said another retiring Republican, Rep. Mark Sanford of South Carolina, who recently lost his primary election after frequently criticizing Trump.

“What the president just signaled is, ‘I’m not going to be there.’ And therefore I think people will take the cue,” Sanford said. “I think it makes immigration reform that much more unlikely.”

Republican Rep. Mike Simpson of Idaho said lawmakers who are counting on Trump to provide a presidential nudge should reconsider. “He changes so frequently that anybody who depends on that, I think, is in trouble,” he said.

Last year, not long after House Republicans stood in the Rose Garden with Trump celebrating passage of their replacement for former President Barack Obama’s health care law, he mocked their legislation as a “mean” bill. After Congress approved a budget deal to end a government shutdown in February, Trump turned on lawmakers and threatened to veto it.

“Republicans should stop wasting their time on Immigration until after we elect more Senators and Congressmen/women in November,” Trump tweeted early Friday. “Dems are just playing games, have no intention of doing anything to solve this decades old problem. We can pass great legislation after the Red Wave!”

Officials said the president’s haphazard actions seem to have less to do with campaign politics – or even his own standing with voters – than ego. He’s frustrated with Congress and the media, particularly after the flop of the Republican health care overhaul last year, which made him wary of fully embracing legislation before it passes.

What is unclear, though, is whether the president realizes the moderate Republicans he is alienating by shunning their immigration overhaul are among those most endangered in the midterm elections.

“No one has more to lose in November than the president does when it comes to the majority in the House, because if this majority flips over to being a Democrat, there will be a big push for impeachment,” said Rep. Bradley Byrne, R-Ala., who opposes the immigration bill.

House Republican leaders have made it clear they do not expect the immigration bill to pass, but have little choice but to press forward and keep a promise made to moderate Republicans.

One leading architect of the bill, Rep. Carlos Curbelo of Florida, holds out hope that the bill can be revised to gain support.

“What we need from the president is for him to sign a good immigration bill, and he and his team have indicated that he will. That’s all we need,” Curbelo said.

Besides, said Rep. John Faso, R-N.Y., a supporter of the bill, maybe Trump will change his mind again: “Just wait a few hours, the tweet will be different.”