Updated at 11:20 p.m. ET

Rep. Darrell Issa, R-Calif., announced he will not seek re-election, adding to a record number of House Republicans heading for the exits ahead of the 2018 midterms — perhaps seeing the writing on the wall of a possible wave election.

There are now 31 Republicans who will not seek re-election in November: 19 who are retiring outright and another 12 who are running for higher office. And that list is is expected to grow in the coming weeks.

The last time a party had nearly that many members retire during a midterm year was in 1994 when 28 Democrats left, and the GOP subsequently took back control of Congress in the Republican Revolution. Now, it's Republicans who find themselves in the opposite and unenviable position. Just one year into his term, President Trump has record low approval ratings, congressional Republicans have had few legislative achievements save for the tax overhaul they passed last month, and Democrats seem more fired up than ever to issue a rebuke to the GOP at the ballot box this year.

"There's no question when you look at these midterms that the Democratic base is more intense than the Republican base, and that offers some significant challenges to Republicans in holding the House, and retirements in marginal seats like [Rep. Ed] Royce's don't help," said former Rep. Tom Davis, R-Va., a former National Republican Congressional Committee chairman.

On Monday, Royce, a California Republican, added his name to the retirement roster. The House Foreign Affairs chairman is now one of eight committee chairmen who are calling it quits, but his seat is in one of three open districts held by a GOP member that President Trump lost last election, making it a top Democratic opportunity.

Democrats need 24 seats to flip control of the House, and history, in addition to other promising signs, is on their side. Going back to World War II, the president's party loses an average of 28 seats in his first midterm election — and none of those happened with a president's approval rating as low as Trump's currently sits.

According to the latest RealClearPolitics average, Democrats have an 11-point edge in the generic ballot, on track with where they need to be to pick up the seats to take control, even with the advantage Republicans have from gerrymandering and the pattern of Democratic voters clustered in urban areas.

"Vulnerable House Republicans would clearly rather call it quits than stand for reelection with a deeply unpopular agenda hanging over their heads," said Democratic Congressional Campaign Committee spokesman Tyler Law. "The total number of Republican retirements and the importance of the open seats to the overall battlefield are a huge problem for Speaker Ryan's imperiled majority."

For Democrats who saw things go the other way the last time the House flipped in 2010, the retirement announcements are a bit of deja vu — but the good kind.

"Retirements, especially from competitive seats, are a harbinger of the cycle to come," said Democratic strategist Jesse Ferguson. "Some members do truly retire to spend more time with their families, but many retire rather than face voters in a tough re-election in a bad climate. And you'd rather go out on top than go out as a loser."

Ferguson was working at the Democratic Congressional Campaign Committee eight years ago when they began to see their own avalanche of members in competitive seats decide to leave.

"I remember sitting there seeing Democrats who had told us they were going to run announce that they were retiring, and we knew full well it was partially driven by concerns about their own re-election prospects," Ferguson recalled.

Royce could fall into that category. The California Republican had insisted last year he was on board for another term, but then abruptly changed his mind on Monday. His Orange County district — which used to be solid Republican territory — voted for Democratic nominee Hillary Clinton over President Trump in 2016 by more than 8 points. It's now one of the most vulnerable seats in the country, and follows a pattern of longtime GOP-held suburban seats sliding away from Republican control.

There have already been other retirements for Republicans in diverse, swing seats that Clinton won, inducing heartburn for party leaders. Rep. Illena Ros-Lehtinen's Miami district is all but gone for the GOP; Trump lost it by 19 points to Clinton, making it the most Democratic-leaning seat held by a Republican.

Republicans could struggle to hold onto Washington Rep. Dave Reichert's suburban Seattle district, too. Clinton eked out a three point victory there and it sits dead in the center of the Partisan Voter Index (PVI), a measure by the Cook Political Report of the competitiveness of districts. Rep. Frank LoBiondo's southern New Jersey district is another that Clinton carried by 4 points that Democrats have a very good shot at picking up.

And there's likely to be another opening on the board if Arizona GOP Rep. Martha McSally, as expected, announces her bid for Senate on Friday. Clinton carried her Tucson district by about five points in 2016.

There are other open seats created by retirements that give Democrats opportunities as well, particularly in marginal seats. Trump won the districts of both retiring Reps. Dave Trott, R-Mich., and Charlie Dent, R-Pa., by 6 points or less and the PVI gives them both a rating of R+4.

If other seats in that same category come on the board, that could be where the GOP House majority is made or broken. Those members on Democrats' watch list include New Jersey Reps. Leonard Lance and Rodney Frelinghuysen as well as New York Rep. Peter King. All three voted against the tax overhaul package which greatly reduced the deduction for state and local taxes (SALT), which are particularly high in their states.

However, Davis said even with the House numbers climbing, he doesn't see it as a "tsunami of retirements" — yet.

He and other Republican strategists pointed out that many members are simply running for other offices, which means they don't see the environment as completely toxic next year. Plus, many House committee chairmen, regardless of who controls the chamber next year, would have been forced to give up their gavel due to term limits — and going back to being a backbencher is never appealing.

The vast majority are leaving behind safe Republican seats that will undoubtedly stay in GOP hands and have simply been in Congress for decades and are ready to go, like Reps. John Duncan, R-Tenn., Lamar Smith, R-Texas, and Bob Goodlatte, R-Va. However, many of them undoubtedly remember when they were in the minority and know it isn't an appealing prospect.

Still, Republicans remain optimistic in their ability to defend their competitive seats, and are even more buoyed by the passage of the tax cuts last month, which will help energize their base as, they believe, people will see more money in their pockets.

"We've had numerous quality candidates announce in these open seats, and we're confident they will remain in Republican control," said NRCC spokesman Jesse Hunt.

Democrats have some headaches of their own due to retirements. Fifteen of their members aren't seeking re-election to the House, with seven retiring outright and eight running for higher office.

The most problematic opening is created by Rep. Tim Walz's decision to run for Minnesota governor — Trump carried his district by 15 points. Another seat Democrats will have to fight to hold on to is that of Rep. Carol Shea-Porter, D-N.H., who is retiring outright and is the only Democrat leaving who sits in a district Trump won.

Two Senate candidates that give Democrats excellent shots at picking up seats in Arizona and Nevada have, in turn, created headaches for the House committee. Trump carried Rep. Jacky Rosen's southern Nevada seat by one point, and she only barely beat a weak GOP candidate. In Arizona, Rep. Kyrsten Sinema's decision to run for Senate gives Republicans an opening as well — her seat has a PVI of only D+4, however, Clinton easily won the district by 16 points.

And the decision by Rep. Ruben Kihuen, D-Nev., not to seek re-election amid sexual harassment allegations adds another seat they have to defend in a district Clinton only carried by five points.

Those openings on their own are just one reason many Democrats are urging caution in assuming that there is a massive blue wave forming in November like Democrats had before they took back the House the last time.

"I think there's a good chance," said one Democratic member of Congress. "But to get a wave you got to have a very energetic base for the minority party, and you have to have a suppressed base for the majority party." The member pointed to the fact that upwards of 80 percent of Republicans remain supportive of President Trump, higher than GOP support for President George W. Bush ahead of the 2006 midterm elections when Democrats swept to power. "So in an off-year, when just the base is kinda turning out, I think we can pick up a lot of seats."

Copyright 2018 NPR. To see more, visit http://www.npr.org/.