Washington (CNN) UPDATE: This story was updated Wednesday after California Rep. Darrell Issa announced his retirement.

Democrats searching for the 24 seats they need to flip to win control of the House in the 2018 midterms are increasingly seeing openings in districts where tough-to-beat Republicans are retiring.

So far, 32 House Republicans are either retiring or running for other offices. That's more than double the 15 Democrats who are not running for re-election.

Rep. Ed Royce, the chairman of the House Foreign Affairs Committee, and Rep. Darrell Issa, the former House oversight chairman, joined the list of retirees this week. Rep. Martha McSally is set to become the 33rd departure when she launches her Senate campaign Friday.

Why do retirements matter so much? Because it's much harder for the challenging party — in 2018's case, the Democrats — to beat an incumbent than to win an open seat. The Cook Political Report has useful data that lays this out

There are several Democratic retirements that have given Republicans big openings, too: Minnesota Rep. Tim Walz's departure could allow the GOP to take a district that President Donald Trump won by 15 points in 2016. Rep. Carol Shea-Porter's New Hampshire district has a long history of switching hands. And Rep. Jacky Rosen's decision to run for the Senate in Nevada opens up a competitive seat there, as well.

But in a wave election — the kind that crashed the 1994, 2006 and 2010 midterms and could be building amid a backlash to Trump's presidency — the Republican-held seats will be among the most hotly contested in the country.

Here are the 10 biggest Republican retirements so far, ranked in order of how significantly they shook up the House battleground map:

1. Rep. Ed Royce -- California 39th

Welcome to Orange County, the most important region on the 2018 House map, with five Republican-held seats in the area that Democrats are targeting.

Royce cruised to re-election by 14 percentage points in 2016 despite Democrat Hillary Clinton winning his district by 9 points.

So the announcement Monday that Royce, the chairman of the House Foreign Affairs Committee, will retire means Democrats suddenly have a huge opportunity to capitalize on this educated, diverse, suburban region's rapid move leftward over recent elections.

Not having to face the $3.5 million Royce had in the bank makes it even better news for Democrats. Republicans have several options here with local and state-level experience. But six Democrats are in the race, including Gil Cisneros, a $266 million lottery winner, and Andy Thornburn, a health insurance executive who has loaned his own campaign $2 million.

2. Rep. Dave Reichert -- Washington 8th

Another tough-to-beat Republican incumbent is leaving a seat where Democrats have had a recent advantage.

Clinton won Reichert's suburban Seattle district by 3 points and former President Barack Obama bested Mitt Romney here by 2 points in 2012.

Republican interim state senator and perennial candidate Dino Rossi will try to hold the seat against one of the nine Democratic candidates currently running.

3. Rep. Frank LoBiondo -- New Jersey 2nd

LoBiondo won his seat in the 1994 wave election and rarely slipped below 60% in his re-election campaigns.

So even though his district went for Trump by 5 points in 2016, his retirement is a major opportunity for Democrats -- particularly since Obama carried the district by 8 points in 2012.

The leading Democratic candidate is one of the party's top recruits in the nation: state Sen. Jeff Van Drew, who is likely too conservative for many progressives but is well-known in the district.

4. Rep. Darrell Issa -- California 49th

Issa barely held on in 2016, winning by a little more than 1,600 votes -- less than a percentage point -- in a district Clinton carried by 8 points.

Headed into a potential rematch against Douglas Applegate (or one of the three other Democrats in the race), Issa announced Wednesday that he'll retire instead.

Southern California is diverse, educated and rapidly moving leftward. Republicans will turn to Diane Harkey, a member of the state Board of Equalization. But this coastal district that stretches from Orange County to San Diego was already poised to be one of the toughest for the GOP to hold in the country. That looks even tougher now.

5. Rep. Martha McSally -- Arizona 2nd

McSally faced a difficult race against Democrat Ann Kirkpatrick, who ran for the Senate against John McCain in 2016, in a Tucson-based district Clinton won by 5 points. So she opted for a run for retiring Sen. Jeff Flake's seat instead.

McSally is set to announce her candidacy Friday, Republicans familiar with her plans told CNN.

Republicans are optimistic about Lea Márquez-Peterson, the Tucson Hispanic Chamber of Commerce president. But with McSally yet to make her plans official, the field might not yet have fully developed.

6. Rep. Iliana Ros-Lehtinen -- Florida 27th

This is a huge opportunity for Democrats to pick off a district that already ought to be in their column: Clinton won it by 20 points — even as the popular Ros-Lehtinen was re-elected by 10 points — and Obama won by 7 points in 2012.

In what looks increasingly like a wave election, the math suggests this Miami-area district should be among the first to flip, no matter who emerges from both parties' crowded primary fields.

7. Rep. Dave Trott -- Michigan 11th

This district — which loops awkwardly around Detroit in a prime example of gerrymandering — is Republican-leaning, but not by much. Trump won here by 4 points in 2016 and Romney won by 5 points in 2012.

Among the five Democratic candidates, technology executive Suneel Gupta raised $500,000 in the last quarter and state Rep. Tim Greimel is also running.

Perhaps the most interesting Democrat is Haley Stevens, who ended 2017 with $650,000 on hand. She's the former chief of staff of the Obama administration's Auto Task Force -- a powerful resume line in the district that's home to Chrysler.

Former Rep. Kerry Bentivolio, who lost his seat to Trott in a 2014 primary, is among the Republicans running now that Trott is retiring.

8. Rep. Charlie Dent -- Pennsylvania 15th

Dent's Lehigh Valley clearly leans rightward, thanks to the presence of Democratic voters who are more conservative than most nationally: Trump won it by 8 points, and Romney won it by 3 points.

Northampton County District Attorney John Morganelli joined the Democratic field of six other candidates this week. Five Republicans are also in the race, including state Rep. Ryan Mackenzie.

9. Rep. Lynn Jenkins -- Kansas 2nd

Why is a district that Trump won by 18 points on this list? Mostly because of the presence of former state House Minority Leader Paul Davis as the leading Democrat in the race.

State Sens. Caryn Tyson and Steve Fitzgerald and state Rep. Kevin Jones are among the Republican candidates.

10. Rep. Steve Pearce -- New Mexico 2nd

Pearce's decision to run for governor gives Democrats an opportunity in the southern half of New Mexico, where Trump won by 10 points and Romney won by 7 points in 2012.

This district belongs firmly in the "wave watch" category. If it flips, then Democrats have probably taken the House -- and it's probably not that close.

(Another fit in this category is the Florida 6th, where Rep. Ron DeSantis is running for governor and Democrats are high on Nancy Soderberg, a former ambassador to the United Nations and national security adviser under President Bill Clinton.)

A special election to watch: Pennsylvania 18th

This is a special election to replace Rep. Tim Murphy set for March 18. It's a heavily Republican district that Trump won by 20 points -- but Murphy resigned in disgrace amid allegations that he had an extramarital affair with a woman who he then encouraged to have an abortion.

Democratic 33-year-old former federal prosecutor Conor Lamb takes on Republican state Rep. Rick Saccone. Lamb has said he wouldn't support Rep. Nancy Pelosi for House speaker.

Are Republicans worried? The Congressional Leadership Fund super PAC has opened two offices in the district.

This one -- like a spate of House special elections in 2017 -- looks like it could break optimistic Democrats' hearts. But a Lamb win would send the national GOP into panic mode.

This story has been updated to reflect new developments.