Americans are angry about war and prices, and a president’s second midterm is always difficult — but a series of decisions by President Donald Trump has made Republicans’ Senate majority ever-more vulnerable.

The mark Trump made on the midterms began in the summer of 2024, when he selected former Ohio Sen. JD Vance as his running mate. Since then, the president’s personnel and political decisions have reshaped the Senate battleground from what looked like an impenetrable red wall into a toss-up.

His decision to pick Vance opened up a special election in Ohio that will by the end of this week cost the GOP hundreds of millions of dollars and potentially a Senate seat. Trump’s party-boss mentality in North Carolina, Texas, and South Carolina has further contributed to the yawning battleground.

He endorsed Rep. Mike Collins in Georgia’s Senate race against Democratic Sen. Jon Ossoff — a contest that never really got off the ground after Georgia Gov. Brian Kemp, whom Trump tried to take down in a primary in 2022, passed on the race. Ossoff is favored to win.

And party insiders blame Trump’s tariffs and war with Iran for creating a volatile Senate map that has left seats in red states like Nebraska and Kansas vulnerable. Republicans could hold the chamber — or lose as many as a half-dozen seats on Tuesday.

“Every negative for the GOP this cycle is a downstream effect of Iran,” said a person close to the White House. Republicans can lose up to three seats and still keep Senate control.

Trump’s zeal on the campaign trail is undeniable and he helped direct huge sums of money into the midterms, albeit later than many Republicans would have liked. He spent the last weekend capping off a breakneck fall campaign schedule stumping in Iowa and Kansas, two places where the GOP was strongly favored to win a few months ago but now is struggling.

“President Trump has a 97% endorsement record and he remains the best turnout machine for our base voters,” said Republican National Committee press secretary Natalie Baldassarre.

Trump is hosting nearly 20 tele-rallies in the last days of the campaign, in addition to his almost-daily travel for rallies over the past month. The White House has sought to emphasize the administration’s accomplishments on issues like the border, crime, and US investment and manufacturing, as they look to encourage low-propensity voters to turn out for Republicans.

“President Trump is the unequivocal leader, best messenger, and unmatched motivator for the Republican Party, and he is committed to maintaining Republican majorities in Congress,” White House spokesman Davis Ingle said.

But his moves to reshape the party in his image over the past two years also often showed little regard for conventional political risk assessment.

Trump created two special elections with decisions to put Vance as his vice president and Marco Rubio, a popular former Florida senator, as his secretary of state.

On Tuesday, Democrats are underdogs in Florida after nominating democratic socialist Angie Nixon to face appointed GOP Sen. Ashley Moody. The race is more competitive than expected but not a five-alarm fire like Ohio, where Vance’s replacement, Sen. Jon Husted, R-Ohio, is in a precarious position.

The longtime preference of Husted, a former lieutenant governor, was to run for governor, according to multiple people familiar with internal party deliberations. He was instead appointed to the Senate and Vivek Ramaswamy ran for governor; Ramaswamy now trails Democrat Amy Action. A campaign spokesperson for Husted said that he “is completely focused on retiring Sherrod Brown once and for all this Tuesday.”“Public polls in the national media have gotten Ohio wrong every election cycle since 2016,” one Ohio GOP operative argued.

A year after selecting Vance, Trump tangled with Sen. Thom Tillis, R-N.C., over healthcare cuts included in the tax cut law, prompting Tillis to retire. Tillis told Semafor afterward that Trump’s backing might not have helped anyway: “I’m not sure that his endorsement would have helped me in a general election in North Carolina next year, to be quite candid.”Trump quickly cleared the field for former RNC Chair Michael Whatley, avoiding a costly primary, but Whatley’s candidacy crumpled against his Democratic opponent, former Gov. Roy Cooper.

Senate Majority Leader John Thune said in an interview last summer that Tillis was “established” and “knew how to run and win in North Carolina,” while also praising Whatley as a candidate who could win with Republicans united behind him. In recent weeks, however, Republicans essentially ceded the race as the battleground map expanded to places like Texas.

Thune and many sitting GOP senators had hoped to convince Trump to back Sen. John Cornyn, R-Texas. But Trump gave Cornyn the cold shoulder after the Texas Republican’s remarks questioning Trump’s viability heading into the 2024 election cycle.

Trump instead backed Texas Attorney General Ken Paxton, who won the nomination. Paxton is trailing in most polls while receiving the most air support of any candidate.

“Texas was an unforced error, I think, on the part of the president,” Cornyn told Semafor in previously unreported comments earlier this year. “We spent a lot of money trying to win the nomination, but the promise was that if I won the nomination, then that would be a layup in November. That’s all off the table.”Had they been nominated again, Cornyn and Tillis may have had tough races too, but most Republicans now believe they would have been closer.

Paxton could still win, but the race undoubtedly opens up a shocking vulnerability in Republicans’ red wall. So too in South Carolina. After the death of his longtime ally, Sen. Lindsey Graham, R-S.C., Trump backed Sen. Darline Graham, R-S.C., who most senators thought would be an interim senator.

Instead, Trump helped launch Graham’s run for a six-year term, even as several other prominent Republicans mounted their own campaigns. Graham defeated conservative Rep. Ralph Norman, R-S.C., in the primary runoff this summer with Trump’s help, but Graham is now in a toss-up race with Annie Andrews, who is outraising her by astronomical margins.

Norman barely lifted a finger to help Graham and the race has gotten closer and closer, prompting intervention from the Senate Leadership Fund, a GOP-aligned super PAC.

“There’s an incredibly divided Republican Party in South Carolina,” Andrews told Semafor in September. She added that polling clearly showed “that Darline was going to be easier for me.”Republicans say some of Trump’s impact has been positive. Trump helped draft top GOP recruits like Michigan Senate nominee Mike Rogers, directed tens of millions of dollars into battlegrounds, and gave Sen. Susan Collins, R-Maine, distance from him in a state where he’s not popular.

Republicans also credit Trump with clearing primary fields in several crucial races. “The president and his team have been valuable partners in the fight for the Senate majority,” said Joanna Rodriguez, a spokesperson for the National Republican Senatorial Committee. “We will continue working tirelessly as one team.”

Alex Latcham, executive director of the Senate Leadership Fund, said, “Republicans are fully united in our efforts to defend the Senate majority and deliver commonsense solutions for working families.”Trump was very careful with his narrow House majority, even rescinding an administration position for Rep. Elise Stefanik, R-N.Y., over fears it could cost them a seat. His relationship with the Senate has been much more tempestuous because of his political decisions.

Adding further strain has been Trump’s pressure campaign on the Senate to kill the filibuster, pass voting restrictions, and stomach the tariffs and the Iran war. It’s little surprise that’s all led Republicans to this moment of deep uncertainty.