The 2018 Senate midterm elections presents Republicans with a host of opportunities, but the party has already lost several top-tier candidates to fill the seats.

GOP Reps. Sean Duffy Sean DuffyCNN's Ana Navarro to host Biden roundtable on making 'Trump a one-term president' Bottom line McCarthy blasts Pelosi's comments on Trump's weight MORE (Wis.) and Pat Meehan (Pa.) both recently announced that they’ll run for reelection instead of mounting Senate runs in blue-leaning states where President Trump pulled off upset victories.

Republicans are losing out on potential challengers in safely GOP states, too. Indiana Rep. Susan Brooks Susan Wiant BrooksHouse Democrats' campaign arm reserves .6M in ads in competitive districts Stefanik credits Trump for spike in GOP women running for office DCCC reserves new ad buys in competitive districts, adds new members to 'Red to Blue' program MORE ruled out a run. Montana Rep. Ryan Zinke will likely be confirmed to lead the Interior Department, taking a top competitor out of the mix in that deep red state.

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“The House [members] are generally pretty politically savvy people,” said Kyle Kondik, managing editor of Sabato’s Crystal Ball, a politics website that handicaps elections. “They know midterms are often — not always — bad for the president's party.”

“Trump is off to a historically weak start in terms of his approval. … You got a lot of members of the House who are in relatively safe seats. Maybe they’re making the determination that this might not best year to run for Senate.”

While a few star GOP contenders have bowed out, Republicans are shrugging it off. They point to a deep bench of other credible candidates who they believe are just as capable of taking on vulnerable Democrats.

Republicans argue that it’s too early to tell whether Trump’s performance or midterm election dynamics are impacting House members’ decisions against Senate bids. While the first few weeks of his administration have been chaotic, they say voters in states like Pennsylvania and Wisconsin may view the president differently than those within the Beltway.

“It’s kind of hard to see the ‘this caused that,’" because what is “happening in the states is a lot different than what’s happening in our view,” said a national Republican operative.

Democrats will mostly be on defense in 2018, with 10 of their seats up in states Trump carried. But even with a rough map, the party of the incumbent president historically sees losses during the first two years in office.

“The folks that are saying they don’t want to run right now, I’m happy they're doing it,” said a national Democratic operative focused on Senate races.

“They’ve got deep benches in a lot of these states, so we’re prepared for them to put forward credible challengers, and we’re getting ready for tough races all over the map,” the operative said, adding that all of the vulnerable Democratic incumbents are “experienced, dedicated, hardworking.”

In Pennsylvania, Meehan was seen as a top contender, but his spokesman noted the prominent role on the House Ways and Means Committee that he would have to give up to run.

Meehan had nearly $2 million to mount a credible run and, while his district is considered only GOP-leaning, he’s won each reelection with at least 60 percent of the vote since he came to Congress in 2010.

Even though Trump ended up pulling off a razor-thin victory in Pennsylvania, Meehan had called on him to end his campaign after a 2005 tape was leaked in which Trump bragged about groping and kissing women without consent. Meehan said he would instead cast a vote for Trump's running mate, then-Indiana Gov. Mike Pence Michael (Mike) Richard PenceControversial CDC guidelines were written by HHS officials, not scientists: report Former DeVos chief of staff joins anti-Trump group Scott Walker helping to prep Pence for debate against Harris: report MORE.

GOP strategists in the state say it was disappointing to see Meehan pass on a Senate bid, but they say other “serious” contenders are eyeing the seat to take on Democratic Sen. Bob Casey Robert (Bob) Patrick CaseySecond GOP senator to quarantine after exposure to coronavirus GAO report finds brokers offered false info on coverage for pre-existing conditions Catholic group launches .7M campaign against Biden targeting swing-state voters MORE.

“I think you’ll see several serious contenders emerge over the course of the next few weeks,” said Charlie Gerow, a Pennsylvania GOP strategist.

Casey, who is seeking his third term in the upper chamber, is considered a tough competitor to beat by both parties. Still, Republicans believe he has shifted from being a moderate to a “hard left” Democrat over the years.

“I think there is a growing feeling that Bob Casey is vulnerable and that the opportunity to capture that seat is very real,” Gerow said. “It will be an uphill battle, but it’s one that is increasingly seen as winnable.”

State Sen. Rick Saccone has already filed to run for the seat. There are other GOP House members from Pennsylvania who could run, including Reps. Mike Kelly and Charlie Dent.

In Wisconsin, Duffy passed on a promotion, citing his family. He has eight children, and GOP strategists in the state stress that his decision was based on family reasons rather than political calculations.

Duffy, an early backer of Trump, was seen as a top challenger and some possible candidates were waiting on him to make a decision. But the Wisconsin congressman has recently made some controversial statements while defending Trump’s travel ban.

“If he’s going to not run, it is helpful for him to step aside early so things coalesce around someone else because there are other talented people who could be good candidates,” a Wisconsin Republican operative said. “It’s helpful he made that clear early.

Wisconsin strategists say Eric Hovde, a businessman who unsuccessfully ran in the GOP primary for Senate in 2012, is a “clear option” to take on Democratic Sen. Tammy Baldwin Tammy Suzanne BaldwinKeep teachers in the classroom Cher raised million for Biden campaign at LGBTQ-themed fundraiser Democrats seek balance in backing protests, condemning violence MORE. Other names that have been floated include businessman and Marine veteran Kevin Nicholson, state Senate Majority Leader Scott Fitzgerald and a few state legislators.

Baldwin is running for her second term and recently scored a spot on Senate Democratic leadership. She served in the House from 1999 to 2013 before she was elected to the upper chamber as the first openly gay senator.

This is her first time on the ballot during a non-presidential year, which makes Republicans argue that she rode the wave of former President Obama’s 2012 reelection. And with Trump eking out a win in the Badger State, they believe Baldwin’s politics don’t match those of the states’ voters.

“Tammy is understood to be so deeply vulnerable,” the GOP operative said. “Tammy won with tidal wave turnout thanks to Obama in 2012. She’s not going to have that level of turnout in 2018.”

In Indiana, Brooks ruled out a run, but other House members have been showing interest in challenging Democratic Sen. Joe Donnelly Joseph (Joe) Simon DonnellyEx-Sen. Joe Donnelly endorses Biden Lobbying world 70 former senators propose bipartisan caucus for incumbents MORE. GOP Rep. Luke Messer is “seriously” considering a Senate run and GOP Rep. Todd Rokita also wouldn’t rule it out.

And in another deep red state, behind the scenes maneuvering helped Democratic Sen. Jon Tester Jonathan (Jon) TesterPence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Filibuster fight looms if Democrats retake Senate MORE lose a formidable opponent in Montana. Zinke was being groomed to take on Tester in a state Trump won by more than 20 points. Instead, the president tapped him to helm the Interior Department.

So far, no other candidates in Montana have emerged.

A number of other House GOP members are still sitting on the sidelines as they consider waging Senate bids, including GOP Reps. Kevin Cramer (N.D.), Ann Wagner (Mo.), Evan Jenkins (W.V.) and Barbara Comstock (Va.).

With a little under two years to go and a nascent presidential administration, Kondik said lawmakers may bide their time and assess the political environment in the coming months. He also noted that House members who initially passed could still reverse course over the next two years like then-Rep. Cory Gardner Cory Scott GardnerAirline job cuts loom in battleground states House approves bill to secure internet-connected federal devices against cyber threats Congress needs to finalize space weather bill as solar storms pose heightened threat MORE in 2014 and Sen. Marco Rubio Marco Antonio RubioFlorida senators pushing to keep Daylight Savings Time during pandemic Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll Intelligence chief says Congress will get some in-person election security briefings MORE running for reelection in 2016.

“I think you could have a situation where there are a number of members on both sides who are trying to take the temperature of the environment over the next many months,” Kondik said.

“It’s possible for someone who says no now will decide later to do it.”