It may be July of 2021, but the 2022 midterm elections are already in full swing, with contenders announcing their candidacies, fundraising well underway, and campaign ads already running.

With the Senate currently split 50-50 between the two major parties but the Democrats holding a razor thin majority due to the tie-breaking vote of Vice President Kamala Harris through her constitutional duty as president of the chamber, Republicans merely need a net gain of one seat to retake the Senate.

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The National Republican Senatorial Committee (NRSC), the campaign arm of the Senate GOP, sees opportunities to flip blue seats red in Arizona, Georgia, Nevada and New Hampshire.

But the GOP’s also playing plenty of defense this election cycle, defending 20 of the 34 seats up for grabs. And that includes five seats left open by Republicans who are retiring — two of which are in the crucial battleground states of North Carolina and Pennsylvania.

But they may have to defend an open seat in a third key swing state.

Here’s a look at the five senators – four Republicans and one Democrat – who have not yet spelled out if they’re running for reelection next year.

Sen. Ron Johnson, R-Wis.

Chairman Ron Johnson, R-Wis., talks with a reporter before a Senate Homeland Security and Governmental Affairs Committee hearing on Dec. 16, 2020. (Tom Williams/CQ-Roll Call, Inc via Getty Images)

Sen. Ron Johnson has yet to say if he’s seeking a third term in the 2022 midterms. Johnson vowed in 2016 to only serve two-terms, but he’s left open the possibility of running again. 

"I'm undecided," he told reporters a month ago during a Milwaukee Press Club virtual event. 

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Johnson also said he felt no pressure to decide anytime soon, noting that he didn’t launch his first Senate campaign in 2010 until seven months before the election.

Democrats are targeting the GOP-held seat in Wisconsin, which President Biden narrowly captured in last November’s election.

Sen. Chuck Grassley, R-Iowa

Sen. Chuck Grassley, R-Iowa, speaks during testimony from Supreme Court nominee Judge Amy Coney Barrett on the third day of her confirmation hearing before the Senate Judiciary Committee on Capitol Hill on Oct. 14, 2020 in Washington, D.C. (SUSAN WALSH/POOL/AFP via Getty Images)

There’s less GOP concern over longtime Sen. Chuck Grassley, who’s also yet to say if he’ll run for reelection next year.

Republican insiders say it’s likely that Grassley, who’s 87 but who recently showed off his physical prowess by taking on Sen. Tom Cotton of Arkansas in a push-up competition, will run again in 2022.

And in a state that former President Trump carried by eight points last year, there’s less concern among Republicans of holding the seat if Grassley decides to retire.

Sen. John Thune, R-S.D.

Sen. John Thune, R-S.D., speaks during a Senate Transportation subcommittee hybrid hearing on transporting ​a coronavirus vaccine on Capitol Hill, Thursday, Dec. 10, 2020, in Washington. (Samuel Corum/The New York Times via AP, Pool)

Sen. John Thune, who as Senate minority whip is the No. 2 Republican in the chamber, has not said yet if he’ll run in 2022 for a fourth six-year term.

Thune is next in line to lead his party in the chamber if Senate GOP leader Mitch McConnell, the longtime senator from Kentucky, retires in the coming years. The senator has said he’s dong the necessary things to prepare for another run, but hasn’t made any formal announcement.

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Thune incurred the wrath of the former president during the final days of the Trump presidency for publicly acknowledging that Biden won the 2020 presidential election, and he’s currently facing what appears to be a long shot primary challenge from the right in the solidly red state of South Dakota.

Sen. Lisa Murkowski, R-Alaska

Sen. Lisa Murkowski, R-Alaska, speaks during a Senate Energy and Natural Resources Committee business meeting in Washington on Thursday, March 4, 2021. (Caroline Brehman/CQ-Roll Call, Inc via Getty Images)

Sen. Lisa Murkowski has also yet to file for reelection.

Former President Trump has vowed to campaign against Murkowski if she runs in 2022, as she was one of seven Republicans in the Senate to vote to convict Trump in his impeachment trial earlier this year, following the deadly Jan. 6 insurrection at the U.S. Capitol by right-wing extremists aiming to disrupt congressional certification of Biden’s election victory.

Trump has endorsed Republican candidate Kelly Tshibaka, who’s challenging Murkowski. And the Alaska GOP has endorsed Tshibaka’s campaign too.

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But Murkowski has the support of the NRSC, which backs Senate GOP incumbents running for reelection. And earlier this year the Senate Leadership Fund (SLF), which is aligned with McConnell, announced that it was endorsing Murkowski for reelection.

Murkowski is no stranger to fierce opposition from her own party. When she was running for reelection in 2010, the senator lost the GOP primary to Tea Party challenger Joe Miller, who was backed by former Alaska Gov. Sarah Palin — then at the pinnacle of her political influence. But Murkowski, following her primary defeat, launched a write-in campaign and won the general election.

Sen. Patrick Leahy, D-Vt.

Sen. Patrick Leahy on Capitol Hill in Washington, D.C., on May 1, 2019. (MANDEL NGAN/AFP/Getty Images)

Sen. Patrick Leahy, who was first elected to the Senate in 1974, has yet to announce whether he’ll seek another term in next year’s midterm elections.

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"I don’t even think of it and I really put it out of my mind until the winter before," the 81-year-old Leahy told reporters in Vermont last month.

If Leahy, chair of the powerful Appropriations Committee, runs and wins a ninth term, he would surpass the late Sen. Robert Byrd of West Virginia to become the longest-serving senator in history by the end of his term.