The people with the most power to drive the 2018 congressional agenda, especially after the tumultuous several weeks ahead, are neither members of the Capitol leadership nor the occupant of the Oval Office.

Whatever President Donald Trump wants to get done, however hard Paul D. Ryan and Mitch McConnell work to assist him, whether Nancy Pelosi and Charles E. Schumer decide to collaborate on or confront the Republican program — none of that will matter as much as the actions of just two folks who’ve never even run for federal office.

They are, of course, the premier in Pyongyang and the sleuth on Sixth Street: the North Korean leader Kim Jong Un and the special counsel Robert S. Mueller III.

Each has the capacity to upend the best-laid plans of the most functional and politically unified government, and the Trump-Hill GOP alliance is far from it. Given the fragility of that always transactional relationship, the capital’s power dynamic could be deeply destabilized by either the dictator or the prosecutor at the metaphorical press of a button. That’s because a nuclear weapon or an unsealed indictment could have sufficient force to transform what Washington is focused on through Election Day if not beyond.

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Of course, there’s no surefire way to predict when, or even whether, Kim might launch a missile or Mueller might file charges. But waiting for and worrying about such impending crises will likely become the lawmakers’ dominant activity, perhaps as soon as March.

If either materializes, it will be the only story that really matters this year — surely to the course of history and probably to the midterm campaign dynamic — whatever meager and mostly belated legislative accomplishments get cobbled together this winter.

Trump and the GOP high command convened at Camp David over the weekend to plot an agenda designed to stabilize their currently precarious political position. However much progress might get made toward achieving any agreed-upon goals, a global calamity or a constitutional showdown would totally eclipse those efforts. 

And how Republicans respond to Trump’s behavior at such a watershed moment would decide their own electoral fates.

Starting from behind

The second session of the 115th Congress really gets started this week. The House reconvenes after a fortnight away, while senators are returning following an essentially lost first week back, a skim of snow having limited their work to a single sub-Cabinet confirmation vote that got skipped by a dozen senators.

Before they even start, the members are way behind — their almost default state of affairs in recent years. And so the weeks bracketing Trump’s first formal State of the Union address Jan. 30, his best opportunity to define the year in policymaking on his terms, must be devoted to mopping up the leftovers from the policymaking impasses of the year already past.

No fewer than 10 high-profile items that were scheduled for resolution in 2017 instead comprise the cache of kicked cans crowding the agenda for starting 2018.

They must agree on spending levels for the military and thousands of domestic programs, which they were supposed to do before the fiscal year  began; make a go or no-go choice on the border wall that’s topped Trump’s presidential wish list for a year; decide whether to shield from deportation nearly 800,000 immigrants who arrived illegally on the arms of their parents, which the president asked Congress to do in September; reauthorize medical coverage for 9 million children, which lapsed that same month; and legislate to stabilize health insurance markets, which Republicans started promising last July.

Plus, they’re committed to revamping the rules for policing sexual harassment on the Hill, which have seemed glaringly outdated since October; delivering recovery and relief funding for a string of natural disasters that began last June; updating foreign intelligence surveillance powers as well as the national flood insurance program, both of which had deadlines in December; and deciding to extend or end a collection of tax breaks that lapsed New Year’s Day because they hadn’t been addressed during last year’s tax debate.

Unlike that tax cut, which was entirely an exercise in muscular majority partisanship, all of those matters will require some measure of bipartisan collaboration — at least in the Senate, where the legislative filibuster will give the Democrats leverage over the GOP.

Only when all of that is resolved can the president and the Republican leadership focus exclusively on marketing their shared proactive goals for the year, all of which already qualify as legislative long shots.

The top aspiration looks to be fulfilling Trump’s still not fully formed plan for generating $1 trillion in infrastructure improvements in the next few years, perhaps with one-fifth of the money in direct federal outlays — a level of new spending likely to turn off as many small government Republicans in Congress as it entices economic-pump-priming Democrats.

Ryan’s new top goal, a shrinking of the “welfare state” in the name of post-tax-cut budgetary discipline, looks to get postponed at the behest of rank-and-file members staring defeat in the eye once swing voters and independents turn against them. The same goes for the GOP’s wish to seriously dismantle regulations imposed on Wall Street after the financial calamity of a decade ago.

Heading for a showdown?

None of those anxieties will merit much attention, though, if Mueller or Kim so boldly confronts Trump that he’s unable to deflect his most consequential tormenters with only a characteristic fusillade of defensively offensive rhetoric.

To date, a critical mass of congressional Republicans has decided to remain impressively impassive when confronted with the most outlandish, combustible and reality-challenged presidential behavior they’ve ever known.

Their collective rationalization has been boiled down to this: The country had ample opportunity to evaluate Trump’s suitability for the most powerful job in the world and, despite his manifest limitations, the country chose him anyway. Under the “elections have consequences” principle, it’s incumbent on members of Congress from the president’s party to work with him notwithstanding their own misgivings — not undercut him or undermine the democratic process by imposing their own assessments for those of the electorate.

One year of working with him is now behind those Republicans senators and House members, and they have a single significant achievement to show for it in the form of the tax code rewrite.

The opportunity to further leverage their ostensible unity before the midterm election is slim. For Trump, his marriage of convenience to Republican conservatives is not likely to produce any more worthy legislative offspring. For the Hill’s GOP bosses, their ability to make Trump a useful engine pulling the government to the right has stalled.

And so the time is ripe for Mueller or Kim, and the crises they could foist upon the president, to make 2018 the year when Trump at least begins to be abandoned by his adopted party.

If the special counsel brings more indictments against senior administration officials — or if Mueller details evidence of improprieties by Trump himself, even without calling the open-for-debate question of whether a sitting president may be prosecuted — then the legislative branch will hold its collective breath until Trump decides how to respond.

If he contests the validity of the charges, and especially if he orders the dismissal of Mueller, Capitol Hill will effectively suspend normal operations while dozens of vulnerable GOP lawmakers confront the career-defining question of whether breaking with the president is in their own self-interest.

And if North Korea tests a hydrogen bomb anywhere inside the U.S. territorial waters of the Pacific, or tempts a confrontation with a launch designed to be too close for comfort, the capital’s attention will be riveted on how Trump reacts.

If he retaliates with questionable justification, Republicans will have to decide whether to stand with him nonetheless. And if he asks Congress for an authorization to unleash military might, they’ll be compelled to decide whether to entrust their own survival — not just political, but temporal — to a commander in chief with Trump’s temperament empowered to wage the world’s first nuclear war.