A Senate nuclear showdown is looking almost inevitable this month.

Decisions about the timing of the confrontation’s start, and what its nominal cause will be, look to be settled by the end of the week. After that, the outcome is unclear. But there’s really only a binary choice, between compromise and calamity.

A single confrontation over a filibustered nomination could lead to negotiated settlement that somewhat recalibrates the majority’s power at the expense of the minority’s rights. That is essentially what happened the last time the phrase “nuclear option” took hold of the Senate, eight years ago.

Alternately, the standoff could escalate until a parliamentary explosion destroys this year’s small remaining measure of legislative collegiality and alters the essential atmosphere of the Senate for years to come. That has not happened in modern times.

Senate Majority Leader Harry Reid hopes to settle on his next move at a closed-door Democratic Conference lunch on Thursday. To that end, one of the Nevada Democrat’s baseline tasks will be to determine whether 51 of the group’s 54 members are prepared to vote for any of what he’s been threatening.

At the moment, it appears as if he has no margin for error.

Two Democrats, 34-year stalwart Carl Levin of Michigan and 10-year veteran Mark Pryor of Arkansas, look to have peeled away from the effort altogether. Both remember from earlier periods in their careers, when the Republicans were in charge, that oppositional tactics are the lifeblood of the minority, and both have concluded that “turnabout is fair play” is a political tenet that would be applied to punish the Democrats mercilessly the next time the GOP has the majority, especially if there’s also a Republican president.

Senate Minority Whip John Cornyn made that clear when he took to the floor Tuesday to warn of the consequences of “breaking the rules in order to change the rules.” It was the Republican sound bite summary of any effort to use a simple-majority procedure to end the de facto supermajority requirement for advancing presidential nominees toward their confirmation votes.

“What comes around goes around, and someday the shoe will always end up on the other foot,” the Texan said. “Majorities are fleeting, and that fact should counsel caution and prudence.”

Levin is among the five most senior Democrats, each of whom has been around the Senate long enough to know 16 years in the Senate minority. But only one of them, Patrick J. Leahy of Vermont, plans to remain past the next election, when the GOP has a solid shot at recapturing control of the chamber.

Leahy has not tipped his hand, but his interest in institutional precedents and prerogatives as the new president pro tem, and his interest in nominees for the federal bench from the party’s top seat on the Judiciary Committee, suggest a likely reluctance to upending the filibuster system.

At the other end of the seniority table, however, are 32 Democrats (three-fifths of the caucus) who have become senators since the start of 2007 and who have only known majority power. Their collective clamor for doing away with the old dilatory order is helping drive Reid toward the precipice.

If he’s confident he has the votes to back up his threat, Reid has the option to give the current system a couple more opportunities to work as designed, or to set off the nuclear countdown clock before that.

The slow-burn approach would involve setting up test votes — on motions to invoke cloture, and thereby limit debate — on the nominees for the two remaining vacancies in President Barack Obama’s second-term Cabinet: Thomas E. Perez for Labor secretary and Gina McCarthy to run the EPA.

Although there are objections to both from many conservatives, the Republican leadership has not served notice of a caucus commitment to blocking them. Forcing votes on Perez and McCarthy would be Reid’s way of testing whether Minority Leader Mitch McConnell fervently wants to engage in the battle right away by forming a unified GOP blockade, or whether he is willing to signal at least a brief de-escalation by permitting his troops to vote however they please.

Partly because a decent-sized GOP bloc still believes in the concept that a president should be given wide latitude to assemble the executive branch senior team he wants, the 60-vote threshold for both nominees would be surpassed comfortably. (Putting up the Cabinet nominees first would also create a thin patina of bipartisan collaboration that could help open a period of negotiation to forestall a showdown.)

But that same attitude of Republican leeway does not exist when it comes to nominees for the semi-autonomous regulatory agencies, especially not the National Labor Relations Board or the Consumer Financial Protection Bureau. Once Reid moves to advance Obama’s choices for those agencies, the procedural pin is out of the bomb.

McConnell was explicit Tuesday in saying all 46 senators on his side would unite to prevent up-or-down confirmation of Obama’s three NLRB nominees if for no other reason than their collective taint for having taken seats on the board under recess appointments in January 2012. The constitutionality of that move will be reviewed by the Supreme Court in the coming term after the D.C. Circuit Court of Appeals ruled a president may only make such appointments when the Senate is between sessions of a Congress.

Richard Cordray is at the helm of the CFPB because of a recess appointment that’s under a similar cloud, which is why Obama has formally asked for a confirmation vote that would wipe away questions about the legitimacy of his work.

But the GOP has rolled out a different reason for promising a filibuster. No one of either party should be confirmed to lead his new agency, 43 have said in a letter, until the organizational structure — established under the 2010 Dodd-Frank law — is reconfigured to put the power in the hands of a bipartisan panel rather than a single chairman.

Note that none of this has anything to do with judges, which was the cause of the 2005 nuclear showdown. That fight’s been put off for another day. But the locus of it comes into view Wednesday, when Leahy’s Judiciary panel holds its confirmation hearing for Patricia Ann Millett, the veteran litigator who’s arguably the least controversial of Obama’s three picks for vacancies on the D.C. Circuit.