(CNN) On Wednesday, California Rep. Darrell Issa announced his retirement from Congress, the latest senior Republican to walk away from Washington amid mounting fears that a Democratic wave -- fueled by President Donald Trump's terrible approval ratings -- is headed toward Washington this fall.

Issa's retirement comes hot on the heels of his California colleague Ed Royce's decision to call it quits earlier this week. Both Issa and Royce represent districts that Hillary Clinton carried over Donald Trump in 2016. Of those 23 districts nationwide, four Republicans who hold them are retiring while a fifth -- Arizona's Martha McSally -- i s expected to run for Senate

Royce, who sits atop the House Foreign Relations Committee, is the 9th(!) committee chairman to announce he or she will be leaving the House in 2018.

Issa and Royce are the 31st and 32nd Republicans in the House to announce he won't be running again in 2018; Democrats have just 15 open seats, by contrast.

There are, as always, mitigating circumstances. California could be a political killing field for Republicans in 2018 given that the state's primary system may well mean that two Democrats -- and no Republicans -- are at the top of the ticket fighting to be the next governor.

"There is going to be a GOP congressional wipeout in California in 2018," predicted Steve Schmidt , a California Republican who managed John McCain's 2008 presidential campaign. "Turnout will be driven by the governor's race which will be between two Democrats. This further depresses GOP turnout."

And, Royce, like several other committee chairmen calling it quits, is term-limited out of his chairmanship because of internal House GOP rules. Some retiring members, like Diane Black of Tennessee, are running for higher office. (Black is running for governor, a race where she is considered one of the early favorites.)

But if you take a step back from any individual retirement and look at the big picture, here's what you are left with: Veteran Republicans -- especially those in potentially competitive seats -- are opting to head for the exits rather than remain in a Congress where, if the GOP retains control this November, they would be positioned to continue to wield considerable influence.

Issa and Royce joins Rep. Ileana Ros-Lehtinen of Florida and Dave Reichert of Washington as Republican House members abandoning House seats Clinton won in 2016. . There's also a handful of other seats -- Charlie Dent's in Pennsylvania, Dave Trott's in Michigan, Frank LoBiondo's in New Jersey -- where Trump won but the underlying political realities of the districts suggest Democrats will make a major play to win them this fall.

In each of those cases, the retiring members are simply walking away from Congress. They aren't running for another higher office. They aren't leaving to enter the Trump administration. They don't have cushy, high-paying jobs lined up. They are just leaving.

Why? Some of it has to do with the fact that the so-called governing wing of the Republican Party -- of which most of the names above are affiliated with -- is losing steam in the nation's Capitol. Trump's takeover of the GOP during the 2016 campaign has pushed the governing end of the Grand Old Party to the fringes. Confrontation, not compromise, is now the prized goal of congressional Republicans.

Which means if that you came to Washington to get things done -- which lots of Republicans did -- you aren't having any fun at this point. And, given Trump's penchant for firing on his own troops, there's no telling if and when a Republican member of the House could be on the wrong end of a tweet from the President of the United States.

Another major factor in these retirement decisions is the increasingly dire prognosis for Republicans this November.

Trump's approval ratings have been mired in the high 30s -- at best -- for the last few months; a Gallup weekly tracking poll released Monday showed Trump's approval at 37% and his disapproval at 58%.

The average seat loss for the president's party in midterm elections since 1962 when the president's approval rating is under 50%? Try 40 seats. (Democrats need only a 24-seat gain to reclaim the majority.)

There's plenty more history here -- and none of it is good for Republicans. In the last century, the average seat loss for a president's party in his first midterm election is 23 seats. There have only been three midterm elections in the past 100 years -- 1934, 1998 and 2002 -- in which the president's party has not lost seats in a midterm.

While the history is daunting, the on-the-ground results over the past year might be even scarier for Republicans. In a series of special House elections in 2017, Democratic candidates consistently overperformed Clinton's showing in these districts. At the state legislative level, Democrats kept winning state legislative seats previously held by Republicans

Then, in the last two months of the year, Democrats won two high-profile races: the Virginia governor's race and the special election in Alabama to replace now-Attorney General Jeff Sessions. In each of those races, Democratic base turnout soared while the Republican base was less enthused to turn out.

Combine those results, the electoral history of first term, midterm elections, Trump's dismal approval numbers and the fact that not much gets done in Washington these days, and you have a hugely toxic mix for Republicans.

The twin retirements of Issa and Royce are an acknowledgment that their chances of winning in November were deteriorating -- and they'd rather go out on their own terms. They aren't the first vulnerable Republicans to pack up their things in the face of the wave building out in the ocean right now. And they certainly won't be the last.