Washington (CNN) Two California House Republicans headed for the exits this week as a blue wave builds ahead of this year's midterm elections.

The retirements of Reps. Darrell Issa and Ed Royce -- as well as Arizona Rep. Martha McSally announcing Friday a run for US Senate -- brought the list of GOP House members who won't return next year to 33.

The retirements have at times surprised Republican operatives -- coming with little advance notice. Royce, for example, had insisted in November he would run for re-election, and had raised $3.5 million.

In a political environment in which a Quinnipiac poll found Democrats with a 17-point advantage in the generic congressional ballot -- which asks voters simply whether they'd rather be represented by a Democrat or a Republican -- more retirements, GOP aides acknowledge, could be coming.

Here are three groups of Republicans whose retirements could further shake up the House map.

1. Northeastern suburbanites

New Jersey Reps. Leonard Lance, 65, and Rodney Frelinghuysen, 71, represent intensely competitive districts. Frelinghuysen, in particular, worries Republicans, and was added to Democrats' "Red-to-Blue" program this week. After voting (like Lance) against the GOP tax bill , he could find himself with few allies in Republican leadership. Already, another New Jersey Republican in a similar position -- Rep. Frank Lobiondo -- has decided to retire.

Pennsylvania Rep. Patrick Meehan, 62, passed on a Senate run against Democrat Bob Casey to seek re-election in his suburban Philadelphia district. It's another prime Democratic target: Clinton won the seat by 2 points in 2016, and the gubernatorial election in Virginia showed just how dramatically the suburbs have swung in Democrats' direction since the presidential contest.

And in New York Rep. Peter King, 73, is high on the list of Republicans whom Democrats would like to see retire.

2. The California targets

Orange County is ground zero in Democrats' bid to take control of the House -- with four Republicans representing districts that Hillary Clinton won.

Already, two of the four -- Reps. Ed Royce and Darrell Issa -- announced their retirement this week. The other two, Reps. Mimi Walters and Dana Rohrabacher, are squarely in national Democrats' crosshairs. Walters told supporters in a fundraising email Thursday evening that she is "100% committed to winning this fight."

The 70-year-old Rohrabacher, in particular, has been dogged by his cozy ties to Russia. House Majority Leader Kevin McCarthy once (jokingly, he said) speculated out loud that Rohrabacher is being paid by Vladimir Putin.

On Thursday, yet another California-related twist emerged: The Hill reported that while Issa won't run for re-election in his own district, he's considering a run in neighboring Rep. Duncan Hunter's district, according to unnamed sources. Hunter is in a safer seat for Republicans -- but congressional ethics investigations have made him a target, as well.

"If I was to blow up in the air, then he would be running for it. If I was to blow up, then he would run for the seat," Hunter told The Hill . "If I blow up, yes. Why wouldn't he run for my seat if I was to blow up in the air?"

Hunter's office followed up later in the day with a statement from the congressman saying he is "100% running for reelection."

3. Senior Republicans

The Democratic Congressional Campaign Committee released a 16-person "retirement watch list" that nailed five of the key GOP retirements so far. There are several other notable names included on the list -- largely included because of their age and tenure.

At 84, Alaska Rep. Don Young is the longest-tenured House member. Could a Democrat win his at-large seat in a state that favored Trump by 15 points? The party has won statewide before, including former Sen. Mark Begich winning in 2008, another landmark year for Democrats. Alaska also has an independent governor in Bill Walker.

Rep. Fred Upton of Michigan surprised Republicans in November by dropping a Senate campaign to run for re-election in his perennially competitive Kalamazoo-based House district. One factor that has him on House retirement watch lists: At 64 and in office since 1987, Upton hit the House's term limits and had to give up the Energy and Commerce Committee gavel.

In his district west of Miami, Rep. Mario Diaz-Balart was re-elected by 25 points in 2016. Yet Trump only won his district by 2 points -- showing that, were he to retire, it would be competitive territory.

Democrats didn't run anybody against Rep. Pete Sessions of Texas in 2016 -- but probably should have: Clinton won his district by 2 points. The 62-year-old is urging national Republicans to stay away from his suburban Dallas district, saying he can win it on his own.

Rep. Bruce Poliquin, 64, of Maine represents a district that has shifted dramatically in recent years -- favoring President Barack Obama by 9 points in 2012 and then Trump by 10 points in 2016. He's the only Republican left representing a House district in New England.

Other potential game-changers

A number of younger, popular House Republicans represent districts that Clinton won. And while they haven't shown signs that retirement is imminent, they all face tough re-election races.

If any were to announce their retirement, Democrats would be handed a huge pick-up opportunity.

That list includes Colorado Rep. Mike Coffman, Florida Rep. Carlos Curbelo, Texas Rep. Will Hurd, New York Rep. John Katko, Texas Rep. John Culberson, Minnesota Rep. Erik Paulsen, Virginia Rep. Barbara Comstock, Illinois Rep. Peter Roskam, Pennsylvania Rep. Ryan Costello, Kansas Rep. Kevin Yoder, and California Reps. Jeff Denham, David Valadao and Steve Knight.