Four states - Georgia, Kentucky, Idaho, and Oregon, hold primary contests on Tuesday, including several congressional showdowns that could shape key contests in the autumn battle for control of the House.

And in a move that will give Republicans a bit of breathing room as they protect their fragile, razor-thin majority, voters in a California congressional district will choose a successor for former House Speaker Kevin McCarthy.

But potentially grabbing the most attention during a day of coast-to-coast contests will be a county prosecutor primary in Atlanta.

Fulton County District Attorney Fani Willis, who is leading the prosecution of former President Donald Trump in his 2020 election interference case, faces a Democratic primary challenge from attorney Christian Wise Smith, in a rematch of their race four years ago.

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Fulton County District Attorney Fani Willis

Fulton County District Attorney Fani Willis looks on during a hearing in the case of the State of Georgia v. Donald John Trump at the Fulton County Courthouse on March 1, 2024, in Atlanta. (Alex Slitz-Pool/Getty Images)

Willis has faced plenty of scrutiny for her handling of the sweeping racketeering case against Trump's alleged effort to overturn his narrow 2020 election loss to President Biden in Georgia.

Nearly two months ago, a judge ruled that Willis could continue leading the prosecution of the case as long as Nathan Wade - a special prosecutor she appointed and had a romantic relationship with - resigned.

Willis is expected to fend off a challenge in the primary from local attorney Christian Wise Smith, but will likely face more attacks in the general election from Republican Courtney Kramer, a lawyer who interned for Trump.

Fulton County Superior Court Judge Scott McAfee, who is the judge in the Trump case, is also up for re-election and is facing a challenge from attorney and radio host Robert Patillo in a nonpartisan race.

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With no gubernatorial or Senate races on the ballot this year in Georgia, the state's 14 congressional districts will take top billing.

Controversial Rep. Marjorie Taylor Greene, a high-profile Trump ally, is running unopposed in the GOP primary in Georgia's 14th Congressional District, in the state's heavily conservative northwestern corner.

Four Democrats are facing off in the race to take on Greene in November.

Republican Georgia Rep. Marjorie Taylor Greene

Representative Marjorie Taylor Greene, a Republican from Georgia, speaks to members of the media following the third vote on the first session of the 118th Congress in the House Chamber in Washington, DC, US, on Tuesday, Jan. 3, 2023. (Al Drago/Bloomberg via Getty Images)

Trump is taking sides in the GOP primary race for the open seat in Georgia's 3rd District, in central-western Georgia. The former president is backing former adviser Brian Jack, who is facing off against five rivals in the far-right field in the solid red district.

Two incumbent Democrats in re-drawn districts in metropolitan Atlanta - Reps. Lucy McBath and David Scott face primary challengers. So does Republican Rep. Barry Loudermilk.

In Oregon, it's a battle between the progressive and establishment wings of the Democratic Party in two congressional primaries.

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Two candidates are facing off in the race to succeed longtime Democratic Rep. Earl Blumenauer in the heavily-blue Portland anchored 3rd Congressional District.

And in the 5th District, a crucial swing district stretching south from suburban Portland that will likely help determine if the GOP can hold on to its majority, Democrats will choose a nominee to take on GOP Rep. Lori Chavez-DeRemer, who narrowly won two years ago.

Meanwhile, Republicans will choose nominees in the state's 4th and 6th Districts, where the GOP will aim to flip blue seats to red seats in November.

To Oregon's south, in California, there's an all-Republican special Congressional election to fill the remainder of McCarthy's term in the heavily red 20th District, located in the southern portion of the state's Central Valley.

McCarthy at podium sulking

Rep. Kevin McCarthy, R-Calif., speaks to reporters hours after he was ousted as Speaker of the House, Tuesday, Oct. 3, 2023, at the Capitol in Washington.  (AP Photo/J. Scott Applewhite, File)

Vince Fong, a state lawmaker and former McCarthy district director, has the backing of the former House speaker as well as Trump's endorsement as he faces off against Mike Boudreaux, a local county sheriff.

The winner will fill the seat for the rest of the year and will instantly boost by one the GOP's current 217-213 majority.

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Fong and Boudreaux will face off again in November for a full two-year term.

Republican Reps. Thomas Massie of Kentucky (left) and Marjorie Taylor Greene of Georgia call for the ouster of House Speaker Mike Johnson  during a press conference  outside the US Capitol in Washington, DC, on May 1, 2024. (Photo by Brendan SMIALOWSKI / AFP) (Photo by BRENDAN SMIALOWSKI/AFP via Getty Images)

Republican Reps. Thomas Massie of Kentucky (left) and Marjorie Taylor Greene of Georgia call for the ouster of House Speaker Mike Johnson  during a press conference  outside the US Capitol in Washington, DC, on May 1, 2024. (Photo by Brendan SMIALOWSKI / AFP) (Photo by BRENDAN SMIALOWSKI/AFP via Getty Images) (hoto by BRENDAN SMIALOWSKI/AFP via Getty Images)

The race to watch in Kentucky is the Republican primary in the 4th Congressional District, where Rep. Thomas Massie - who backed Florida Gov. Ron DeSantis in the GOP presidential primary and pushed along with Greene to oust current House Speaker Mike Johnson - is facing a challenge from Trump supporter Eric Deters.

Kentucky and Oregon will also hold presidential primaries on Tuesday. 

Get the latest updates from the 2024 campaign trail, exclusive interviews and more at our Fox News Digital election hub.