Rep. Lamar Smith, a 16-term member of Congress from Texas, announced his retirement at the end of his term.

Politico:

Smith's 21st District, which takes in parts of Austin and San Antonio as well as rural counties to the west, has been solidly Republican in the past. Mitt Romney won nearly 60 percent of the vote there in 2012.

But Trump got 52 percent to Hillary Clinton's 42 percent in the district in 2016, and several Democrats saw an opportunity to challenge Smith before he announced his retirement. One Democrat, veteran Joseph Kopser, outraised Smith in the third quarter and has over $219,000 in his campaign account.

"The people of Texas are losing a dedicated public servant and skilled legislator, but we are confident they will select another conservative Republican like Chairman Smith who shares their values," said National Republican Congressional Committee Chairman Steve Stivers (R-Ohio).

Smith is just the latest in a slew of GOP House members who have announced their retirement.  Not including Smith, here's a list of Republicans who are vacating seats for a variety of reasons:

Retiring Senate Republicans

1. Sen. Bob Corker, R-Tennessee
2. Sen. Jeff Flake, R-Arizona

Retiring House Republicans

1. Rep. Charlie Dent, R-Pennsylvania
2. Rep. John Duncan, R-Texas,
3. Rep. Jeb Hensarling, R-Texas
4. Rep. Lynn Jenkins, R-Kansas
5. Rep. Sam Johnson, R-Texas
6. Rep. Dave Reichert, R-Washington
7. Rep. Ileana Ros-Lehtinen, R-Florida
8. Rep. Dave Trott, R-Michigan

House Republicans who have resigned or will resign

1. Rep. Jason Chaffetz, R-Utah
2. Rep. Tim Murphy, R-Pennsylvania
3. Rep. Pat Tiberi, R-Ohio

House Republicans running for another office

Senate:

1. Rep. Lou Barletta, R-Pennsylvania
2. Rep. Marsha Blackburn, R-Tennessee
3. Rep. Evan Jenkins, R-West Virginia
4. Rep. Luke Messer, R-Indiana
5. Rep. Todd Rokita, R-Indiana

Governor:

1. Rep. Diane Black, R-Tennessee
2. Rep. Raul Labrador, R-Idaho
3. Rep. Kristi Noem, R-South Dakota
4. Rep. James Renacci, R-Ohio
5. Rep. Steve Pearce, R-New Mexico

Republicans were already swimming upstream in 2018, given the history of majority party losses in an off-year election.  How difficult does it make the GOP effort to keep control of the House?

FiveThirtyEight:

After August's long congressional recess, retirement is suddenly looking pretty good to many Republican members of Congress. On Monday, Rep. Dave Trott of Michigan's 11th Congressional District became the latest in a string of Republicans to step away from competitive U.S. House seats. The rapid-fire retirements have quickly given rise to the narrative that the unpopularity of President Trump – which threatens to hang like a lead weight around Republican candidates' necks in 2018 – is scaring Republicans straight off the ballot.

At first glance, the list of retiring congresspeople fits that narrative pretty well. Of the 24 members who have thus far decided not to run for reelection in 2018,1 16 are Republicans and 8 are Democrats. But it's not immediately clear that electoral endangerment is the reason for that lopsidedness.

It will take more than the unpopularity of President Trump to boot the Republicans from their position in the majority.  It takes viable candidates, lots of money, and a platform to run on.  Republican gains in the 2010 and 2014 off-year elections were the result of hard work finding suitable candidates; raising gobs of cash; and, most importantly, having an agenda that resonated with the voters.  Democrats have, so far, failed in each of those categories, although they still have time to rectify the situation.

A rash of retirements doesn't necessarily signal a wave election, and (predicted) waves don't necessarily spur more retirements. Instead, the pattern of retirements appears to be generational as well as about representatives responding to the political environment. Republican retirees have outnumbered Democratic ones in most election cycles since the mid-1990s; for the 20 years before that, Democrats usually retired at higher rates than Republicans. Members of big freshman classes like 1994's and 1974's have to hang up their spurs sometime.

That said, the most fundamental takeaway is still this: Retirements from a competitive state or district hurt the party the member belongs to. The reason is simple: incumbency advantage. It's easier, for example, for a Democrat to win in a slightly red district in an open election than to take on a sitting House member. The more Republicans in competitive districts who retire heading into 2018, the more seats Democrats can realistically go after.

And there may yet be plenty more. At 24 announced open seats so far this year, we are still a long way from the 40 or so total that is normal for recent cycles. Keep an eye on how many more retirements are in the offing; the more Republicans who head for the exits, the worse 2018 could be for the party.

The number of retiring members is virtually irrelevant compared to where those retirements take place.  It appears that most of the Republicans retiring or seeking higher office are from relatively safe districts.  The same goes for Democrats.

That may change in the coming months as the electoral landscape takes shape in each and every congressional district.  Barring a 2010-like wave election, the Republican majorities will probably narrow some, but they should be able to hang on to control.