Congressional Republicans have let slip a golden opportunity to make good on their most important and counterintuitive campaign promise of 2018 — covering for President Donald Trump at every mind-numbing opportunity.

They still have half a year to change their collective minds, but for now the GOP is essentially all in on one of the most outside-the-box political strategies of all time: Betting that safe passage for their imperiled majorities requires lashing themselves to a president mired in record low approval ratings, subsumed by self-orchestrated chaos and in the crosshairs of a special counsel.

They had an opening this week to make that tough-to-fathom game plan just little bit easier to execute, by shielding Trump from some of his own worst impulses just as he was starting to thrash them out in public.

The decision by House and Senate leaders not to force some legislative immunotherapy on the president may prove to be brilliant, or at least defensible, with the benefit of hindsight. But in the meantime it’s a bit of a head-scratcher.

The opportunity to administer the inoculation was the omnibus spending bill, covering all discretionary appropriations for the final half of this fiscal year. The medication would have been language effectively preventing Trump from launching a history-altering move that even his staunchest allies agree would be more self-destructive than any of the myriad outré things Trump has done so far: Attempting to fire special counsel Robert S. Mueller III.

Watch: McConnell, Schumer Defend Mueller

But the idea of adding such a rider was dropped early on in the final frenzy of haggling this week, before the final $1.3 trillion package was set to be sealed shut in hopes of enactment before a government shutdown deadline Friday (and the start of a two-week congressional spring break).

The president would have said he abhorred such new restrictions on his executive authority, of course, even while insisting he had no intention of using it.

But, with apologies to the teetotaling president, Hill Republicans could have framed it instead as a sort of “friends don’t let friends drive drunk, or even start drinking when driving late at night is assured” moment. His extraordinarily committed core of GOP accommodators at the Capitol could have insisted they were doing him a favor, shielding him from even the possibility he might someday fall prey to the very least angels of his nature and decide he is compelled to lash out.

If the president really believes he has done nothing wrong, and if the collective view of Republicans in Congress is that they’re willing to take this manifestly untrustworthy president at his word, then the only thing they all have to fear is precipitating a stop-DC-in its-tracks crisis, born of Trump’s frustrations and impatience that the Mueller probe has already sucked whatever little legislative oxygen there was going to be for 2018 out of the capital.

Crossing a line

The venue for helping Trump remained open at the start of the week, right after the president prompted the most intense worry in GOP ranks in months that he was putting Mueller back in his crosshairs. After his own top attorney, John Dowd, called for the special counsel probe to be shut down, the president fully jettisoned his legal team’s months-long strategy of orchestrated deference with a vituperative Twitter string deriding a partisan “witch hunt” based on false premises and orchestrated by a Democratic hack. (Mueller is a lifelong Republican.)

Firing Mueller would, of course, propel the already roiled Trump presidency squarely into Watergate-constitutional-crisis territory. And senior Republicans — fully aware of that history and concerned for their country’s psyche as well as their party’s political fortunes — recognize this would be a political red line the president could not cross and still hope to keep his extraordinary level of congressional fealty intact.

Lindsey Graham of South Carolina, who has perhaps the most unusual frenemy relationship with Trump of any Republican senator, put it bluntly: “If he tried to do that, that would be the beginning of the end of his presidency.”

There was a flurry of discussion in Democratic circles about making such a rider a make-or-break, shut-down-the-government-or-bust condition for getting the Democratic votes necessary to enact the spending bill. But that talk quickly fizzled, in part because Democrats would presumably benefit from a Mueller dismissal and in part because their pushing probably did nothing more than steel the spine of the Republicans to unite behind a flat “no” — even on something that might be in their long-term self-interest.

“The special counsel should be free to follow through with his investigation to its completion without interference, absolutely,” Speaker Paul D. Ryan told reporters Tuesday, the gentlest of brushback pitches from someone who before the election was among Trump’s most powerfully nettlesome critics in the GOP. And then came this vote of confidence in the president’s judgment: “I am confident that he’ll be able to do that. I’ve received assurances that his firing is not even under consideration. We have a system based upon the rule of law in this country. We have a justice system. And no one is above that justice system.”

“I don’t think that’s going to happen, so I think it’s not necessary,” Senate GOP Whip John Cornyn of Texas had said the day before. “I don’t see the necessity for picking that fight right now … I don’t think it’s helpful.”

To review: The president does not have the power to actually fire the special counsel, but he may order his attorney general to do the deed, so long as there is some written justification offered in advance. And since Attorney General Jeff Sessions has recused himself — probably the most bittersweet unintended consequence of being the first GOP senator to endorse Trump way back when — the order would be given to Rod Rosenstein, the deputy attorney general.

To be sure, there are legitimate separation of powers concerns with each of the leading proposals, which surfaced in the Senate last summer during an initial burst of Trumpian agita over the Mueller probe. One, by Republican Thom Tillis of North Carolina and Democrat Chris Coons of Delaware, would set a high bar for dismissal in law and give a panel of judges two weeks to review whether a firing had been warranted. The other, by Graham and New Jersey Democrat Cory Booker, would require a panel of judges to approve a dismissal before it could be carried out.

But those arguments never got aired in recent days, because two stronger and more political rationales carried the day.

First, the GOP decided to try to keep the bill free from all but a handful of politically freighted riders, so the Mueller language joined a pile on the cutting room floor that included proposals to shore up Obamacare’s insurance market, temporarily shield “Dreamers” from deportation, punish “sanctuary cities” and Planned Parenthood, start funding the border wall and jump-start construction of a new tunnel under the Hudson River.

First and foremost, however, GOP leaders concluded once again that their best approach to Trump is to passively permit him to skate along as close to the precipice for as long as he can sustain that dangerous course.