After a month on the sidelines, the 2026 primary season is back with a vengeance.A dozen states from coast to coast hold primaries or runoffs in May, and the results of those nomination contests may ultimately determine the outcomes of Novembers midterm elections, when Republicans will be defending their slim Senate and razor-thin House majorities.Also on the line in some of the ballot box showdowns: President Donald Trumps immense sway over the GOP, as his endorsements in key races will be tested.ONLY ON FOX NEWS: TRUMP WILL 'DELIVER' - RNC CHAIR SIGNALS MIDTERM CONFIDENCE DESPITE 'DOOM AND GLOOM'Indiana and Ohio kick off the action on May 5, with Nebraska and West Virginia holding primaries a week later, on May 12. Louisianas nominating contest follows on Saturday, May 16. Three days later marks the busiest day of the month, with Alabama, Georgia, Idaho, Kentucky, Oregon and Pennsylvania holding primaries. Texas wraps up May with runoff showdowns on May 26.Heres a closer look at some of the top races.The first major test of Trumps grip on the GOP comes in Indiana.Five months ago, Republicans in the GOP-dominated state Senate withstood immense pressure from Trump and his allies and voted down congressional redistricting, which would have given solidly red Indiana two more right-leaning U.S. House seats ahead of the midterms. Seeking retribution, the president endorsed challengers to eight GOP state senators who voted against the redistricting bill.The presidents allies have spent millions of dollars to try to oust the state lawmakers who opposed Trumps redistricting push. Among those in the political fight on behalf of the president are Turning Point USAs political wing and the Club for Growth.WHAT THE LATEST FOX NEWS NATIONAL POLL SAYS ABOUT DEMOCRATS AND REPUBLICANSThe intra-party battle is seen not just as a test of fealty to Trump but rather a fight between MAGA forces and more traditional conservatives for the future of the GOP."Weve got to change those old-style Republicans, put in people who will fight, fight against the Democrat gerrymandering," Club for Growth President David McIntosh told Fox News Digital.McIntosh, a former congressman from Indiana, said "I want to see my state do the right thing."In neighboring Ohio, theres a lot less drama.Vivek Ramaswamy, the multimillionaire biotech entrepreneur and business leader who grabbed national attention during his bid for the 2024 GOP presidential nomination before dropping out and becoming a top Trump surrogate, is all but certain to capture the Republican gubernatorial nomination in his home state. Ramaswamy, who is backed by Trump, will face off in November against Dr. Amy Acton, a doctor and researcher who served as director of the state Department of Health from 2019 to 2020. Acton is unopposed in the Democratic primary. The winner will succeed term-limited GOP Gov. Mike DeWine.DEMOCRACY 26: STAY UP TO DATE WITH THE FOX NEWS ELECTION HUBIts the same story in Ohios Senate primary, where appointed Republican Sen. Jon Husted, a former lieutenant governor, is unopposed in the GOP primary. Former longtime Democratic Sen. Sherrod Brown is expected to cruise to his partys nomination. The winner will serve the final two years of the term of Vice President JD Vance, who stepped down from the Senate after the Trump-Vance ticket won the 2024 presidential election.Once a top general election battleground state, Ohio has shifted to the right over the past decade, with Trump carrying the state by 11 points in the 2024 election. But this year's races for the Senate and governor are expected to be very competitive. And the Senate race is one of a handful across the country that may determine if the GOP holds the majority or if the Democrats flip the chamber.Sen.Bill Cassidyof Louisiana is facing primary challenges from two Republicans: Rep. Julia Letlow and former Rep. John Fleming, who is currently the state treasurer. Trump earlier this year weighed into the race by endorsing Letlow.Cassidy was one of only seven Senate Republicans who voted in early 2021 to convict Trump after he was impeached by the House for his role in the violent Jan. 6 attack on the U.S. Capitol by supporters who aimed to upend congressional certification of former President Joe Biden's 2020 election victory. Trump was acquitted by the Senate.CRUZ WARNS RADICAL DEMOCRATS WILL BURN IT DOWN IF THEY WIN BACK CONGRESSBut since the start of Trump's second term 15 months ago, Cassidy has been supportive of the president's agenda and his nominees.If no candidate cracks 50% of the primary vote, the top two finishers will face off for the nomination in a June 27 runoff election.The third major test of Trumps endorsement power this month is in Kentuckys 4th Congressional District, where Rep.Thomas Massieis facing a challenge from Trump-backed Ed Gallrein.Massie has long been one of Trump's most vocal GOP critics in Congress, repeatedly taking aim at the president over the Epstein files and foreign policy.Trump allies have spent big bucks to boost Gallrein, a former Navy SEAL, and to take aim at Massie.The presidents endorsement is also being tested in Georgias GOP gubernatorial nomination, in the 2026 race to succeed popular conservative Gov. Brian Kemp, who is term limited.Trump has endorsed Lt. Gov. Burt Jones, who is trading fire in a competitive and combustible battle with healthcare executive and mega GOP donor Rick Jackson, who has infused millions of his own money in his bid. Among the others battling for the nomination in a crowded Republican field are state Attorney General Chris Carr and Georgia Secretary of State Brad Raffensperger.Former Atlanta Mayor Keisha Lance Bottoms, who later served in then-President Joe Bidens administration, is the frontrunner for the Democratic nomination. Among the other contenders in the crowded field of candidates are Mike Thurmond, a former DeKalb County CEO and former state Labor Commissioner, and former Republican lieutenant governor turned Democrat Geoff Duncan.Republicans are hoping to flip the U.S. Senate seat up for grabs this year in Georgia. The GOP views first-term Sen. Jon Ossoff as the most vulnerable Senate Democrat seeking re-election this year. But beating Ossoff, who has built a massive war chest, wont be easy in the southeastern battleground state.Making matters worse for the GOP: Theres a nasty primary between major contenders Reps. Mike Collins and Buddy Carter, and former college football coach Derek Dooley, who is backed by Kemp. Trump has remained neutral to date in the Senate primary in Georgia.Longtime GOP Sen. John Cornyn is fighting for his political life as he faces off in a runoff election against state Attorney General Ken Paxton, who is a MAGA firebrand and major Trump supporter.Trump has stayed neutral in the showdown between the two Republican titans in right-leaning Texas.Cornyn narrowly edged Paxton in an early March GOP primary that also included Rep. Wesley Hunt, but with no candidate topping 50%, Cornyn and Paxton advanced to the runoff.The winner of the runoff will face off in November with Democratic nominee James Talarico, a state representative and rising Democratic Party star who hauled in an eye-popping $27 million in fundraising the first three months of this year.Democrats are confident, and Republicans are concerned, that if Paxton wins the GOP nomination, Republicans will have a harder time in the general election holding the seat. And similar to the Senate race in Ohio, the showdown in Texas is one of a handful across the country that may determine if the GOP holds the majority.