The race pitting a candidate endorsed by President Donald Trump against Congressman Thomas Massie, a rare Republican critic of Israel, has become the most expensive House of Representatives primary contest in the history of the United States.

The avalanche of spending, totalling more than $34m by Monday, according to official records, highlights the significance of the elections that could oust one of the few Republican opponents to the war with Iran.

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In the final stretch of the campaign ahead of Tuesday’s vote, Massie has sought to highlight the oversized role of pro-Israel groups – including the American Israel Public Affairs Committee (AIPAC) – in the race.

He said the election will be a “referendum on foreign policy” and whether pro-Israel lobby groups will be able to “bully” members of Congress.

“You can tell that I’m ahead in the polls, and they’re desperate,” Massie told ABC News on Sunday.

“That’s why they’re sending the secretary of war to my district tomorrow. That’s why the president’s losing sleep and tweeting about this. That’s why AIPAC has dumped another $3m into my race this weekend.”

Trump has been incessantly bashing Massie on social media, and in an unusual move, Pentagon chief Pete Hegseth has travelled to Kentucky to campaign for Ed Gallrein, the Navy SEAL veteran challenging the congressman.

Massie has been critical of the unconditional US military aid to Israel and of the country’s abuses in Gaza and Lebanon. He has also helped spearhead the push for the release of government files related to the late sex offender Jeffrey Epstein.

The money

Despite the intensity of the race, the candidates have not raised record amounts of money themselves.

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The bulk of the spending, more than $25.8m, has come from outside groups, known as super political action committees (super PACs).

Super PACs are usually used by special interest groups to spend heavily to oppose or support a candidate without the constraints of legal limits on direct campaign contributions.

Pro-Israel groups and donors have played a central role in the flood of funds and ads directed against Massie, with three groups linked to them spending more than $15.5m in the race, Federal Election Commission (FEC) data shows.

United Democracy Project (UDP), AIPAC’s election arm, has spent more than $4.1m.

The RJC Victory Fund, which is affiliated with the Republican Jewish Coalition, came in with around $3.9m.

MAGA KY has been the largest spender, at $7.5m.

The PAC’s finances have not been made fully public. But available records show that one of the group’s top funders is Paul Singer, a pro-Israel billionaire investor who has also made the largest individual donation to UDP over the past year – $2.5m.

MAGA KY also received funds from Preserve America PAC, a group linked to Israeli-American megadonor Miriam Adelson.

Details of the finances of Preserve America PAC remain unclear for this election cycle. But Adelson donated $106m to the PAC in 2024 to help elect Trump as president.

Trump has openly admitted that Adelson and her late husband Sheldon Adelson have influenced his Middle East policies.

Before the race in Kentucky’s Fourth Congressional District, the most expensive House primary was the 2024 election that ousted then-Democratic Congressman Jamaal Bowman, in which pro-Israel groups, including AIPAC, were also the largest spenders.

The third most expensive primary also involved AIPAC and its pro-Israel allies, who succeeded in helping defeat progressive Congresswoman Cori Bush in 2024.

The Trump factor

Beyond the millions of dollars in pro-Israel spending, Massie needs to survive another potent force in Republican politics – Trump’s wrath.

The US president has all but purged the party of lawmakers who have disagreed with him on major issues.

Most recently, Senator Bill Cassidy – who voted to convict Trump after the January 6, 2021, US Capitol riot – lost his primary to a challenger backed by the US president.

Trump is actively campaigning against Massie. In less than 24 hours between Sunday and Monday, the US president fired off three social media posts berating the congressman, calling him “weak”, “pathetic” and a “bum”.

“The worst Congressman in the long and storied history of the Republican Party is Thomas Massie,” Trump wrote on his Truth Social platform on Monday. “He is an obstructionist and a fool. Vote him out of office tomorrow, Tuesday. It will be a great day for America!”

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However, Massie appears to have a few advantages that other Republican dissidents lacked.

Over the years, the congressman has built a reputation as a combative, principled libertarian and has gained popularity among right-wing commentators.

His campaign directly raised $5.5m, significantly more than Gallrein’s $3.1m, while also receiving outside support from pro-gun rights and libertarian PACs.

Massie has also been endorsed by some of his Republican colleagues, including Congresswoman Lauren Boebert, an outspoken right-wing lawmaker.

And due to the involvement of pro-Israel groups, Massie’s supporters are arguing that the race is not all about Trump, who remains popular amongst Republican voters.

“Why does Trump hate Massie? Is the congressman a secret liberal? Not at all,” right-wing commentator Tucker Carlson said in his newsletter on Monday.

“Unlike nearly everyone else in the Republican Party, Massie has refused to go along with the White House’s abandonment of the America First principles that got the president elected. He is one of the few honest people in politics. Everyone who cares about our country should root for him.”