One of the biggest congressional stories of the decade starts unfolding Monday — not at the Capitol, but across the street.

The Supreme Court will hear arguments in an epic balance of powers battle between the other two branches, one that’s been waiting to happen since George Washington’s time. During the hour, the justices may or may not signal clearly whether they’re going to permit the continued expansive use of the president’s recess appointment authority — or seriously limit its use for the first time.

That second outcome would give the Senate enormously more influence over the leadership of the departments and agencies and the tenor of the federal courts. But if the court rules that way, it will be almost impossible to notice any difference in the power dynamic before the beginning of next year — if then.

It may sound a bit paradoxical, but it’s the “nuclear option” that would guarantee such a delayed reaction.

And during that delay, a new measure of importance would get attached to the midterm elections.

Here’s why. No matter what the court says about recess appointments in a decision expected in June, for the following six months Senate Democrats will still be able to exercise their newly engineered ability to advance all of President Barack Obama’s nominees (unless there’s one for the Supreme Court) without finding 60 votes to overcome Republican opposition.

And if his party holds on to Senate control in November, the same would hold true during Obama’s final two years in office. Even if they are still permitted, he would nonetheless probably have no incentive at all to make recess appointments — which in practice have been used by presidents only to fill vacancies with people they expected the Senate wouldn’t cotton to. If the court reins in or maybe even does away with this long-standing practice, Obama would have no reason to be upset, because his party-mates would still have the post-nuclear power to confirm whomever he wants.

The same would hold true for future presidents whenever their party ran the Senate. So long as the new curbs on the filibuster remain, there will be minimal practical impact from a ruling that declares unconstitutional more than two centuries of recess appointment practices.

The situation could be very different whenever the White House and Senate are in different hands, which has at least sometimes been the case in every administration since Ronald Reagan’s. That is yet another incentive, as if any more were needed, for the Republicans to throw everything they’ve got at gaining the six seats they need to take over in January 2015.

If the status quo is upheld, a GOP majority would have no new ability to stop Obama from returning to recess appointments as a tool for shaping the courts and the bureaucracy during his lame-duck years. But the outcome the party is dreaming about — one that effectively takes away this presidential maneuver — would give majority Republicans the ability to defang the president’s regulatory and judicial crusades without lifting a finger. Since they would set the agenda and control the calendar, GOP senators could take a silently passive-aggressive approach to their “advice and consent” power — by simply ignoring the nominees they don’t fancy, without any fear those people would sneak in anyway using the recess back door.

The stakes for Senate Republicans are so high that the court gave them 15 minutes of argument time. Minority Leader Mitch McConnell will be on hand as their case is presented by Miguel Estrada. (His nomination by President George W. Bush to the D.C. Circuit Court of Appeals failed to advance during seven cloture votes in 2003 — a bellwether early moment in the confirmation wars that climaxed with November’s party-line changes to the Senate rules.)

The court case is an outgrowth of Obama’s decision to test the limits of his power by filling three vacancies on the National Labor Relations Board in January 2012 — a time when the Senate was on a three-week holiday break but was holding lickety-split meetings every three days, at the GOP minority’s insistence, for the express purpose of preventing such recess appointments.

The president argued such pro forma sessions were a sham that shouldn’t constrain him. The 45 Republican senators say the Senate, not the president, gets to decide what constitutes a legitimate meeting. The main plaintiff in the case, a Pepsi bottler in Washington state called Noel Canning, says the ruling against it by the NLRB in a labor dispute with the Teamsters should be tossed out because the board did not have a legitimate quorum.

“The president shall have power to fill up all vacancies that may happen during the recess of the Senate,” says the last paragraph of Section 2 of Article II, and those people may remain on the job until the end of the following session of Congress.

Not only did the D.C. Circuit rule it unconstitutional for the president to make appointments while the Senate is holding pro forma sessions, it also decreed the only permissible time for a president to avoid the confirmation process was after one annual session of Congress ends but before the new one begins. The D.C. Circuit also said recess appointments could only fill vacancies created during that period.

If the Supreme Court upholds all those rulings, the Senate majority would have an easy time making sure the recess appointment window stays open for a president of the other party to use for only a moment every year.

Legal scholars are salivating over the outcome of what they say is the most important separation of powers case in at least two decades. That’s because it not only reflects the most basic argument about constitutional law (whether strict constructionist originalism is most important, or applying precedent and common sense to real world situations) but also could have the effect of theoretically invalidating thousands of decisions by dozens of recess appointees dating back more than 200 years.

But for senators expecting to be around next year, it’s a clear-cut case of power politics.