Two days before registered Republicans voted in the party’s primary election in Kentucky’s 4th Congressional District on May 20, Donald Trump called the incumbent representative, Thomas Massie, “the worst Republican congressman in history”. Massie subsequently lost the primary to a political newcomer with no prior office-holding experience. Ed Gallrein’s not-so-secret weapon was that he had the backing of the US president.Just over a week later, Texas voters were asked to decide whether 22-year Senate veteran John Cornyn should be ousted in favour of the state’s attorney-general, Ken Paxton – who was also endorsed by Trump. Despite all the baggage Paxton carried into the race: an indictment for fraud (charges were later dropped) and an impeachment for bribery, which he denied before being acquitted in the state senate. He has also gone through an acrimonious divorce accompanied by accusations of adultery (which he has also denied), Paxton won the May 26 primary handsomely with more than 60% of the vote. Trump has long threatened to “primary” – back a rival candidate in the upcoming primary election – Republicans who displease him in some way. But with the midterm elections looming in November, we’re seeing this put into practice. And it’s making the conservative “old Republican” wing of his party very nervous.America’s high-profile November elections involve straightforward contests between the nominees of the main parties. But before a candidate can represent their party, they must first win an internal election. These primaries are open to registered party voters (and, in some states, independents)American political parties have no centralised power to simply appoint or protect their candidates. The process is genuinely competitive and, as the current cycle is demonstrating, potentially dangerous for incumbents.For the president to mount a primary challenge is to use a particularly powerful weapon in American political life – it can end a career without the opposing party winning a single vote. On one level, the 2026 GOP primaries are rolling out in the usual manner. However, who is orchestrating them, and why, is worthy of note.According to the Brookings Institute, Thomas Massie drew Trump’s ire not for any ideological deviation from the GOP line, but for opposing a short-term funding bill and for joining a Democrat in calling for the Department of Justice to release the Epstein files.The Texas case has a similar logic. Prior to his decades in the Senate, John Cornyn served as Texas attorney general and sat on the Texas Supreme Court. He is a stalwart conservative by any conventional measure. However, he was associated with a bipartisan gun safety bill in 2022, and he has at times been willing to work across the aisle. His challenger, Ken Paxton, is a Maga true believer who survived a bipartisan impeachment attempt in the Texas senate, largely on the strength of Trump’s support.The pattern extends well beyond these two cases. In one state alone, Trump endorsed challengers to eight GOP state senators who had voted against a redistricting bill, with his allies spending millions in an effort to remove them. The message is clear: vote against the president’s wishes, and he will come for your seat.Electoral gambleThis strategy is inevitably unnerving for the more traditional wing of the Republican Party. Democrats are confident, and Republicans concerned, that a Paxton nomination in Texas will make it harder for Republicans to hold the seat in November. The Democratic nominee, state representative James Talarico, raised a staggering US$27 million (£21 million) in the first three months of the year alone.There is some Trumpian precedent for all of this. In 2017, the 45th president endorsed Luther Strange in an Alabama senate primary. Strange lost to Roy Moore, who then lost the general election to Democrat candidate Doug Jones. The lesson that a rock-solid Republican seat can be lost was clear, although apparently unlearned.Republicans are defending slim majorities in both the House and Senate in 2026. They can afford few losses. Replacing electable incumbents with ideologically pure but extreme and therefore electorally risky challengers, is a strategy that appears to prioritise control over the party above control of Congress.The effect on Capitol HillEven where Trump’s candidates win, the consequences may be destabilising. Some Republicans have acknowledged that Trump’s aggressive involvement in primaries could create complications, not least for members who are no longer worried about reelection. Senators in their final term, for example, might be emboldened to act independently knowing there is no electoral sword hanging over them.But the more immediate effect is silence. Had Massie or Cornyn survived their primary challenge, more members of Congress might have been willing to vote against Trump’s interests. Their defeat sends the reverse signal. When a solid incumbent with a strong conservative record can be unseated for insufficient loyalty, other Republicans in Congress will be watching and calculating.This is how party discipline can slide into something more troubling. It is one thing for a party leader to want to manage unwieldy factions but enforcing one’s authority via electoral intimidation is another matter.What is being challenged in these primaries are the remnants of what the Republican Party once was. This included a coalition of business conservatives, foreign policy hawks, libertarian-leaning figures and traditional social conservatives. Massie represented one strand of that tradition. Cornyn represented another. They have now been treated as enemies of the Maga agenda.Historically, US election scholarship has suggested there may be a tendency in primaries to swing towards the party’s base, and then back in the direction of the median voter for the general election. In this new incarnation of the GOP, the pendulum appears stuck to the right. During primary season, this may be an attractive Maga trait. But come November, not every GOP voter may cast their ballot for a cult of loyalty.Clodagh Harrington does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.