When Donald Trump tried to overturn the results of the 2020 election, some of the clearest and most consistent objections came from the senior senator from South Dakota, John Thune. That was notable because Thune was then serving as majority whip of the Republican-controlled Senate. It is even more notable now because, on Wednesday, Thune was elected to serve as Senate majority leader during Trump’s second presidential term.

While Thune now says he has developed a working relationship with the incoming president, and suggests that the Senate Republican Caucus is “united” behind the new administration’s agenda, he has a very long record of making clear his distaste for Trump’s personal and political excesses—and of being the target of his fellow Republican’s ire.

The most heated clashes between Trump and Thune came over issues that are already surfacing again, as the new majority leader, an institutionalist who prizes the Senate’s ability to check the executive branch, will have to deal with Trump’s determined efforts to undermine the authority of the chamber that must approve his cabinet picks and judicial nominations.

After the 2020 presidential vote, Thune quickly rejected Trump’s attempts to get congressional Republicans to support an effort to overturn the results of an election in which Trump had been decisively defeated. Thune announced at the time, “All the states did everything they were supposed to do in accordance with the law.” When Trump backers stormed the Capitol in a violent attempt to prevent Congress from certifying the Electoral College decision, Thune angrily declared, “We needed to get our work done and this kind of thuggery would not keep us from doing the people’s work.”

While 147 Republicans in the Senate and House voted to sustain objections to 2020 election results from several states, Thune voted with Democrats and a smaller number of Republicans to certify the totals. During and after the struggle, recalled Sioux Falls, South Dakota’s Argus Leader, “Thune also directly denounced Trump for attempting to overturn the results of the 2020 election and his role in the January 6, 2021, riot at the Capitol. He called his involvement and that of the rioters ‘inexcusable.’” (Like his former boss, then–majority leader Mitch McConnell, however, Thune failed to join the courageous Republicans who in February 2021 voted to convict Trump of committing high crimes and misdemeanors.)

It was not the first time that Thune called out Trump. “Thune was also one of the first critics of Trump after the resurfacing of the 2005 Hollywood Access tape, which depicted the New York businessman making crude and sexual comments toward women, reported the Argus Leader. “The news of the tape prompted Thune to say Trump should drop out as the Republican nominee during his first presidential run.”

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But it was always Thune’s refusal to go along with Trump’s challenge to the 2020 presidential election results that stirred the wrath of the once and future president. In December 2020, when Thune said that Trump’s efforts to get the Senate to reject Electoral College results would “go down like a shot dog” in the Senate, Trump was furious. “RINO John Thune, ‘Mitch’s boy,’ should just let it play out. South Dakota doesn’t like weakness,” he declared in December of 2020. “He will be primaried in 2022, political career over!!!”

Trump, as usual, held on to his grudge. On New Year’s Day in 2021, he announced, “I hope to see the great governor of South Dakota, Kristi Noem, run against RINO @SenJohnThune, in the upcoming 2022 Primary. She would do a fantastic job in the U.S. Senate, but if not Kristi, others are already lining up. South Dakota wants strong leadership, now!”

Noem, who is now Trump’s pick to head the Department of Homeland Security, did not mount the challenge. And Thune won his 2022 Republican primary with 72 percent of the vote, before securing reelection in November of that year by a 43-point margin in the overwhelmingly Republican state.

Thune did not rush to reconcile with Trump when it became clear that the president was going to maintain his grip on the GOP. While many Republican senators endorsed Trump’s third presidential bid before the 2024 caucuses and primaries began, Thune pointedly refused to do so—arguing that “it’s clear that running on relitigating the 2020 election is not a winning strategy.”

Eventually, Thune supported a primary challenge to the former president by South Carolina Senator Tim Scott, going so far as to appear at Scott’s May 2023 announcement rally. “Thune is making good on his hopes of turning the GOP away from Trump and presenting a more forward-looking vision for the party,” reported Politico.

Only when it was clear that Trump had secured the support he needed to claim the Republican nomination did Thune finally get on board, albeit with clear misgivings regarding a candidate he had said he had “always been worried” about. Those worries surfaced anew in August, when Thune said during a farm-policy discussion in South Dakota that Trump’s proposed tariffs were “a recipe for increased inflation.”

Now, Thune must determine how to maintain his constitutional-conservative principles, while navigating Trump’s demands for destructive economic and political policies, as well as pressure from the president-elect to approve nominees who might lawlessly advance those policies. That pressure includes a demand that Trump be allowed to go around the Senate to make recess appointments—an approach that the new majority leader has said will only be avoided if cooperation with Democrats is maintained. That’s just one of the many pressure points that will test Thune’s commitment to maintain the Senate’s authority as a check and balance on abuses of executive power. If he buckles, he will not be the first Republican who has done so. If he stands his ground, even minimally, it’s a good bet that Trump will, before long, again be promoting a primary challenge to the senator from South Dakota.