Amidst growing signs of a potentially huge Democratic wave in the 2018 midterm elections, a raft of key Republican would-be candidates are deciding to stay onshore rather than risk drowning in its undertow.

Rep. Kevin Cramer (R-ND) dealt his party the latest in a series of rapid-fire blows to its hopes for holding on to control of Congress on Thursday, announcing that he won’t run against Sen. Heidi Heitkamp (D-ND). Just one day earlier, Rep. Darrell Issa (R-CA) announced he wouldn’t seek reelection in a tough-to-hold swing district. The day before that, Rep. Ed Royce (R-CA) did the same.

Those decisions have Republicans growing even more worried about losing the House and possibly seeing the Senate flip as well.

“It’s a concern. The last thing you want is open seats in a bad year, particularly if they’re in competitive districts,” Rep. Tom Cole (R-OK), a former National Republican Congressional Committee chairman, told reporters Thursday. “We are going to have a very challenging cycle and there’s no question the majority’s at risk.”

Cramer’s decision after both President Trump and Senate GOP leaders pushed to run leaves them without a well-known candidate in a state Trump carried by more than a two-to-one margin. Issa and Royce were two well-known incumbents with huge war chests. One week ago Ohio Treasurer Josh Mandel (R) dropped his bid against Sen. Sherrod Brown (D-OH), leaving the GOP without a well-known candidate there as well, though Rep. Jim Renacci (R-OH) has since announced a bid of his own.

Even Republicans who’d previously dismissed the building narrative around their retirement and recruitment problems admitted that the latest news partly confirmed the pattern.

“Royce and Issa were the members retiring, to me, that started validating the narrative that with all due respect many of you have been pushing for some time,” Rep. Ryan Costello (R-PA), who’s facing a tough reelection battle of his own, told TPM. “My sense was some of that was looking at what the political environment was and making a calculation.”

One Republican who broke the pattern of shying away from a challenging environment could damage their chances in another key race: Deeply controversial former Maricopa County Sheriff Joe Arpaio (R) announced an Arizona Senate bid on Tuesday. (The GOP establishment’s favorite and their best chance to hold the seat, Rep. Martha McSally (R-AZ), launched own bid on Friday).

“It’s concerning, it is,” Sen. Jeff Flake (R-AZ) told TPM about the Senate GOP’s recruiting failures — and Arpaio’s decision to run for his seat. “It’s going to be a tough environment.”

There are four concrete things to watch for in looking for a potential wave election.

The first is polling. Trump’s approval rating is mired in the high 30s, a historic low for a president this early in his term, and Democrats have held a lead in the upper single digits or low double digits in surveys of which party should control Congress (slightly above the seven- or eight-point lead they likely need to flip the House due to structural disadvantages).

The second is candidate fundraising, and Democratic candidates are essentially printing money while most Republicans are struggling.

The third is how off-year elections go. Democrats posted a record-setting margin in the Virginia governor’s race last November before pulling off a shocking upset to pick up an Alabama Senate seat last month that put Senate control up for grabs in a real way by narrowing the GOP majority to 51-49. While Democrats haven’t won any House special elections, their candidates have out-performed their historic numbers in almost every contest so far.

“If you look at Virginia, that’s a bellwether, that’s a scary one in terms of where young voters and women are. That ought to be sobering for Republicans,” Flake said.

The fourth big indicator is incumbent retirements and candidate recruitment. The GOP’s latest setbacks in this category come after a long string of House Republicans in swing seats have announced retirements, and a number of top-tier recruits have passed on Senate bids.

The current number of House GOP retirements — 29 and counting — is outpacing the retirement rate even in previous wave elections (22 Republicans retired ahead of the 2006 Democratic wave, 19 Democrats ahead of the 2010 GOP wave and 27 before the 1994 wave). That includes incumbents in swing or Democratic-leaning districts like Reps. Ileana Ros-Lehtinen (R-FL), Charlie Dent (R-PA), Frank LoBiondo (R-NJ) and Dave Reichert (R-WA).

Senate Republicans are defending just one incumbent in a state Trump lost (Sen. Dean Heller (R-NV)) while Democrats are defending 10 of their own in states Trump won, including in five states he carried by 18 percentage points or more. That means in a neutral year they’d be on offense.

But the GOP has failed to land its initial favorites in states like Montana (state Attorney General Tim Fox), Wisconsin (Rep. Sean Duffy), Michigan (Rep. Fred Upton) and Missouri (Rep. Ann Wagner). Only in Missouri are they particularly bullish about their replacement, state Attorney General Josh Hawley (R). Republicans still hope that Florida Gov. Rick Scott (R) will jump in against Sen. Bill Nelson (D-FL), but he’s dragged his feet on deciding — and if he doesn’t run they’re unlikely to get a serious candidate in another state Trump won last year.

“There are still some efforts underway to recruit in a few of the states, North Dakota being one of them,” Senate Republican Conference Chairman John Thune (R-SD) told reporters Thursday after expressing “disappointment” about Cramer’s decision. “There’s going to be a question of whether the governor runs in Florida, there’s going to be a question of who the candidate is in Ohio … and Montana for that matter.”

On the other side, Democrats have so many House candidates running that their biggest concern in most races is crowded primaries, and they’ve landed top candidates in their few Senate targets — Arizona and Nevada — while putting Tennessee on the map by getting former Gov. Phil Bredesen (D) into the race. Some are also bullish about Rep. Beto O’Rourke (D-TX) making things competitive against Sen. Ted Cruz (R-TX).

And while some other Republicans tried to talk up North Dakota state Sen. Tom Campbell (R), the only announced candidate against Heitkamp, Senate Majority Whip John Cornyn (R-TX) admitted he had no idea who that was after expressing disappointment over Cramer’s decision.

“I honestly have not met that state senator, him or her, whoever it is,” he told TPM.

National Republican Senatorial Committee deputy chairman Thom Tillis (R-NC) argued the party has “a pretty good bench of candidates.” And while he admitted disappointment about Cramer, he said it was still early in the recruiting process.

Sen. Mike Rounds (R-SD) agreed. But he paused for a full five seconds when TPM asked which candidates he was excited his party had landed.

“Man, now you’re going to ask me to pick and choose. I don’t think I want to go there,” he said.

Rounds later mentioned former Minnesota Gov. Tim Pawlenty (R) as a potential top Senate candidate. But the other recruit he brought up unprompted was telling what type of year 2018 might shape up to be for his party.

“The one I think is going to be challenging for Republicans is actually going to be Phil Bredesen in Tennessee,” he said. “He’s going to be a very competitive challenger.”