Pennsylvania’s Democratic lieutenant governor is formally jumping into the race for his state’s open Senate seat, becoming the first major candidate to enter what’s expected to be one of the nation’s most competitive and expensive showdowns in the 2022 fight for control of the Senate.

Lt. Gov. John Fetterman, who a month ago took initial steps toward launching a campaign by setting up an exploratory committee, on Thursday officially filed a statement of candidacy with the Federal Elections Commission, a mandatory step for candidates for federal office.

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The seat is currently held by retiring Republican Sen. Pat Toomey, who announced in October that he wouldn’t run for reelection in 2022, when the GOP will try and win back the Senate majority it just lost.

Pennsylvania Lt. Gov. John Fetterman speaks at a news conference in the governor's Capitol reception room in Harrisburg, Pa., on Jan. 24, 2019. Fetterman is taking definitive steps toward running for the state’s open U.S. Senate seat in 2022. Fetterman said Friday, Jan. 8, 2021, he's exploring a run for U.S. Senate.  (AP Photo/Marc Levy, File)

The 51-year-old Fetterman ‒ who at 6 feet, 8 inches, towers over most people ‒ is anything but your average politician. He sports a tattoo on his left forearm that reads "15014," which is the zip code of Braddock, Pennsylvania, the industrial city near Pittsburgh where he served 14 years as mayor before winning election in 2018 as lieutenant governor. And on his right arm is a tattoo with the dates of the nine homicides in his borough during his tenure as mayor.

The combative Fetterman has frequently butted heads the past two years with the GOP controlled state legislature. He also grabbed national attention as a frequent guest on the major cable news networks, including Fox News, during the post-November election battle over Joe Biden’s narrow victory over then-President Trump in the crucial battleground state.

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The Senate is split 50-50 between the two parties, but the Democrats control the chamber due to the tie-breaking vote of Vice President Kamala Harris. That means Republicans only have to flip one seat in 2022 to regain the majority they lost following the Democratic sweep in last month’s twin Senate runoff elections in Georgia.

But the GOP is defending 20 of the 34 seats up for grabs next year. The difficult map isn’t the only obstacle facing the Republicans. Besides Pennsylvania, they’re also defending an open seat in another crucial battleground state – North Carolina – where Sen. Richard Burr isn’t running for reelection. And two-term Republican Sen. Rob Portman of Ohio announced last week that he’ll retire from the Senate rather than run for reelection in 2022.

There’s also a potential headache in Iowa, where 87-year-old GOP Sen. Chuck Grassley is staying mum so far on whether he will run for an eighth six-year term in the Senate. In the battleground state of Wisconsin, Republican Sen. Ron Johnson hasn’t said if he’ll run for a third term.

But Republicans see pickup opportunities in the key battlegrounds of Georgia and Arizona, where Democrats Raphael Warnock and Mark Kelly won special elections to serve two years in the Senate and are up for reelection in 2022. The GOP also sees possible pickups in Nevada and New Hampshire, where first-term Democratic Sens. Catherine Cortez Masto and Maggie Hassan are running for reelection.

This is Fetterman’s second bid for the Senate. He lost a four-way Democratic primary for the chance to challenge Toomey in 2016. And he’ll have competition once again. State Sen. Sharif Street of Philadelphia has already taken initial steps to launch a campaign. And Reps. Chrissy Houlahan, who represents a suburban Philadelphia congressional district, and Conor Lamb, who represents a district north and west of Pennsylvania, are considering Senate bids.

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The battle for the GOP nomination could also be a slugfest.

Potential candidates include Rep. Mike Kelly, whose district covers the northwest corner of the Keystone State, former Navy Secretary Kenneth Braithwaite, former Rep. Ryan Costello, 2020 congressional candidate Sean Parnell, and Craig Snyder, a former top aide to the late Sen. Arlen Specter.

Fox News' James Levinson contributed to this report.