Some of most competitive Senate races in the country hinge on potential candidates who, for now, are still on the sidelines.

Some of the possible candidates are expected to enter their race but are biding their time. Others are less likely to jump in but could shake up the field if they do.

Here are five states worth watching for a potential candidate to upend a Senate race.

Tennessee

Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE’s (R-Tenn.) retirement, announced Tuesday, opens up the floodgates in Tennessee like only an unexpected vacancy can.

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Rep. Marsha Blackburn Marsha BlackburnDemocrats smell blood with new DHS whistleblower complaint Hillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections GOP senators unveil new bill to update tech liability protections MORE (R-Tenn.) is considered an early favorite to jump in. She’s already announced she’ll take a look at a bid. Blackburn had been the top name floated by the pro-Trump forces looking for a primary challenge to Corker, but most Republicans were skeptical she would run unless Corker retired.

Right now, conservative activist Andy Ogles is the only significant candidate in the primary. But that’s likely to change with an entry by either Blackburn or two other Republicans who have been considering bids, state Sen. Mark Green and former state Rep. Joe Carr.

Green has had a tumultuous few months, ending his bid for governor to accept Trump’s appointment as Army secretary before eventually withdrawing over his controversial comments about gay rights. And Carr ran an unsuccessful primary against Sen. Lamar Alexander Andrew (Lamar) Lamar AlexanderGraham: GOP will confirm Trump's Supreme Court nominee before the election The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda MORE (R-Tenn.) in 2014 that was still closer than expected.

Arizona

President Trump has made it clear that he wants a primary challenger for Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (R-Ariz.), a frequent critic of the president. And his backers have responded in kind — GOP mega-donor and Trump supporter Robert Mercer donated $300,000 to a super PAC backing former Republican state Sen. Kelli Ward’s primary challenge, while Trump allies joined Ward’s campaign in August.

Trump has tweeted encouraging words about Ward’s challenge, but he’s stopped short of endorsing her. And he and allies have continued to speak with other potential primary candidates, including some seen as better general election candidates than Ward.

The talk centers mostly on Jeff DeWit, the Arizona state treasurer and former Trump campaign chief operating officer, and Robert Graham, the former Arizona GOP chairman. Both men, as well as Ward, met with the White House over the summer. Since many state Republicans are skeptical that the ultra-conservative Ward can win in a general election, either man could have a chance at supplanting her as the leading GOP alternative to Flake.

But Ward remains undeterred, particularly as public head-to-head polling shows her leading Flake in a primary by a healthy margin. It’s unclear whether either DeWit or Graham will decide to run, or how a bid could shape the race. But as conservatives continue to call for Flake’s ouster, this race could continue to heat up.

Michigan

The Wolverine State is home to two very different potential Republican candidates.

Detroit singer Kid Rock, whose real name is Robert Ritchie, has captivated the political world with his repeated flirtations with a Senate bid against Sen. Debbie Stabenow Deborah (Debbie) Ann StabenowOVERNIGHT ENERGY: Trump rollbacks could add 1.8 billion tons of greenhouse gas emissions over 15 years: analysis | Intensifying natural disasters do little to move needle on climate efforts | Experts warn wildfire smoke could worsen COVID-19 GAO report finds brokers offered false info on coverage for pre-existing conditions Democrats back away from quick reversal of Trump tax cuts MORE (D-Mich.).

While Ritchie hasn’t announced whether he will actually run, he’s selling shirts adorned with his campaign logo. He has also peppered his recent tour with political statements and diatribes, injecting himself into the political discussion.

While many Republicans believe Ritchie would be a strong candidate thanks to his notoriety, they doubt that he’ll follow through.

That’s not the case with Rep. Fred Upton, the longtime Michigan Republican who has begun sending signals about a potential bid. A source close to Upton told The Hill he’s still considering a bid. It’s one that Michigan Republicans are taking far more seriously than Ritchie’s flirtations with the race.

Upton has been a decades-long fixture of Michigan Republican politics, once serving as the chairman of the House Energy Committee. A likely bid would make him the clear front-runner to win the Republican primary — the remaining candidates have only a sliver of the name recognition and resources that he does. Upton is also believed to have the best shot of beating Stabenow in the general election.

Florida

Republicans are salivating at the chance to knock off Sen. Bill Nelson Clarence (Bill) William NelsonDemocrats sound alarm on possible election chaos Trump, facing trouble in Florida, goes all in NASA names DC headquarters after agency's first Black female engineer Mary W. Jackson MORE (D) in Florida, encouraged after President Trump narrowly won the state in 2016. But no big names have taken the plunge yet for one reason — Republican Gov. Rick Scott.

Scott is far and away the favorite in the Republican primary. His personal wealth means he can jump-start his campaign by self-funding, his name identification is through the roof after two terms as governor, and he’s won two tough statewide races. Maybe most important of all, Trump has publicly called on him to run.

That’s not to say Scott would sail through the general election. His ongoing response to the devastation of Hurricane Irma provides both opportunities to show his leadership and potential risks, especially in his government’s handling of multiple deaths at a nursing home during the storm. Scott’s close political ties to Trump could also prove to be a mixed bag in a purple state.

Scott would immediately enter the race as the strongest Republican candidate, so he can take his time.

Mississippi

If nothing changes, Mississippi Republican Sen. Roger Wicker Roger Frederick WickerHillicon 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 Shakespeare Theatre Company goes virtual for 'Will on the Hill...or Won't They?' Republican Senators raise concerns over Oracle-TikTok deal MORE will have an easy reelection in the deep-red state.

But as former Trump chief strategist Stephen Bannon begins to agitate for more primary challenges from the right to incumbent Republicans, Wicker could find his race changing rapidly.

Bannon’s orbit is chattering about the prospect of backing a primary challenge by Chris McDaniel, the state senator who fell just short of toppling Republican Sen. Thad Cochran William (Thad) Thad CochranEspy wins Mississippi Senate Democratic primary Bottom Line Mike Espy announces Mississippi Senate bid MORE in a 2014 runoff. Bannon has met with McDaniel, who attended a Bannon-headlined rally in Alabama on Monday night ahead of the state’s Republican Senate primary runoff.

McDaniel’s last primary race featured scorched-earth tactics as the party establishment fought the Tea Party. McDaniel later refused to concede the race to Cochran, alleging that illegal votes were cast in the race.

McDaniel already had success in 2014 framing his bid as a fight with the establishment. If Bannon and his allies jump on board to back him, Wicker’s path to reelection could become a lot more complicated.