This has already become a wave election year, because a record wave of departures by House chairmen already guarantees a sea change in the Republican power structure next January.

Even if the GOP manages to hold on to its majority this fall, its policymaking muscle for the second half of President Donald Trump’s term will need some prolonged rehabilitation. And if the party gets swept back into the minority, its aptitude for stopping or co-opting the newly ascendant Democrats’ agenda will require some serious retraining.

That’s because more than a third of the Republicans who began this Congress with standing committee gavels in their hands, eight of the 21, will not be members of the House a year from now.

Neither will three GOP elders who had previously held prominent chairmanships — including California’s Darrell Issa, who gained national notoriety for his relentless grilling of Obama administration officials while running the Committee on Oversight and Government Reform, who announced his retirement Wednesday.

Watch: What’s Next for Seats of Retiring Chairmen?

The narrative behind this exodus has been mainly about experienced political canaries deciding their time is ripe for fleeing the congressional coal mine.

A handful, Issa among them, have reached that conclusion because their own electoral futures look grim. The rest realize their influence will plummet if their side loses the midterm election or (in the case of five of them) because GOP term limits mean their gavels are getting taken away in December no matter what.

Either way, the most lasting consequence will be a brain drain in the top ranks of one party’s committee hierarchy that’s unparalleled in modern congressional history. And that will become even more dramatic if two other chairmen in real re-election danger do not survive for another term.

An edge to professionals

As this president has underscored like no other — and as Oprah Winfrey, this week’s beneficiary of a collective Democratic swoon, will surely learn if she pursues the presidency in 2020 — making federal policy is an occupation that’s far more easily practiced by political professionals, or at least people with some practice at governing.

One of the many reasons why Congress has slid into a dysfunctional decline, and grown so institutionally weak when compared to the executive branch, has been an erosion of expertise that’s attributable in large measure to the shrinking average tenures of lawmakers and their aides.

Fully 55 percent of today’s House members (including 64 percent of the Republicans) have arrived since the current majority took command seven years ago, while three out of every eight senators (19 Democrats and 18 Republicans) have been on the job less than a full six-year-term.

And thousands of staff jobs in members’ personal offices and on the committees change hands every year — the time that many ambitious and smart young people devote to such posts, before making them steppingstones toward other careers, having been sharply curtailed by a Hill salary range that hasn’t kept pace with inflation for more than a decade.

Knowing their limits

The principal counterweight to experience-draining turnover, especially in the House, has been a culture where plenty of lawmakers put a premium on accruing issue know-how and influence along with their seniority on committees, where the bulk of most policy decisions still get made.

Attaining a panel’s chairmanship was for decades seen as the pinnacle of success and a guarantee of power lasting until retirement.

That changed a quarter-century ago, when Republicans waged the campaign that won them control of Congress for the first time in 40 years.

The term limits movement was blossoming in 1994, and so was public sentiment against the hidebound ways that seniority-centric Democrats ran the Hill, and so the GOP promised its members would be limited to three terms in the party’s top seat on any House committee or subcommittee, either as chairman or ranking minority member. (GOP senators imposed slightly less rigorous committee term limits on themselves.)

Top Republican panel posts have turned over routinely ever since, especially on the six-year cycle that began with an almost wholesale changeover in 2000. That year, too, five members decided to depart the House rather than live without a gavel. Four term-limited chairmen did so in 2006, which didn’t help the GOP politically in a year when it lost the House. But only one hewed to the pattern six years ago, right after Republicans won back control.

(Amazingly, of the 68 Republicans first sent to Congress in the “Contract with America” wave of 1994, only eight remain.)

This year, the only term-restricted chairman who has decided to seek re-election is Michael McCaul of Texas. He’s a safe bet to win an eighth term and has his eyes on the top GOP seat on Foreign Affairs, which will come open at the same time he’s forced to hand over the reins at Homeland Security.

Democrats’ gain?

The Democrats have never seriously contemplated such time limits on internal power. As a result, and despite the waves of defeats and subsequent retirements that party witnessed after losing control in 2010, taking over the House in 2019 would mean as many as five committees could once again fall under the gavels of the same members who ran them nine years earlier.

Bennie Thompson of Mississippi (Homeland Security), Louise Slaughter of New York (Rules) and Nydia M. Velázquez of New York (Small Business) are all safe bets to win new terms. Collin C. Peterson (Agriculture) always faces a tough campaign in Minnesota, while federal investigators have been probing whether Robert A. Brady (House Administration) was involved in an alleged scheme by his campaign to pay a 2012 primary opponent in Pennsylvania to drop out of the race.

Beyond that, the Democrats likeliest to take the helms at eight of the most powerful committees were all chairmen of subcommittees on those same panels when the decade began. That’s empirical evidence that, without any comment on ideology, the party would know right away how to operate the levers of legislative responsibility.

The same sort of strong collective muscle memory does not exist in the Republican Conference.

To be sure, leading candidates for four of the top open GOP committee seats include lawmakers with chairman experience.

Beyond McCaul, two of the aspirants to take over at Judiciary are current gavel wielders — South Carolina’s Trey Gowdy, now at Oversight and Government Reform, and Ohio’s Steve Chabot, now at Small Business. Two two who were chairmen earlier this decade, Oklahoma’s Frank D. Lucas (Agriculture) and New York’s Peter T. King (Homeland Security), have a shot at the top spot on Financial Services if North Carolina’s Patrick T. McHenry goes instead for a leadership position. And another former chairman, Missouri’s Sam Graves (Small Business),  is the candidate to beat for Transportation and Infrastructure.

At the same time, at least half a dozen Republicans with realistic chances of attaining committee leadership jobs next year (Gowdy among them) are new enough to the House that they’ve never known life in the minority — a limitation on experience that would especially complicate their efforts if they’re consigned by the voters to managing “loyal opposition” duties in the legislative trenches.