Billionaire Nike founder Phil Knight has made record political donations ahead of the US midterms – and no one seems to know why.

He has given a total of $2.5m to fund a Republican, Knute Buehler, running for governor in the state of Oregon, where Nike is based. This breaks records for individual political donations in Oregon several times over. And pundits suggest he has given an additional $1m via the Republican governors’ association.

Knight’s spokesperson did not immediately respond to a request for comment, and his camp has consistently refused to clarify his motives throughout election season.

Buehler’s campaign is tight-lipped. Asked whether Knight’s donations were made with a specific policy goal in mind, a spokesperson said: “We don’t talk about our donations or donors.”

Asked whether such large donations from a single individual merited transparency, the spokesperson repeated: “We don’t talk about our donations or donors.”

Stepping into the information void, Buehler’s opponent, Democrat Kate Brown, has floated the specter of corruption. Knight is “clearly accustomed to buying whatever he wants,” said a spokesperson. “A lot of Oregonians are wondering if this is an attempt to buy a governor.”

Knight, 80, is Oregon’s richest person by far. His $29bn fortune makes him the 28th richest man in the world, according to Forbes, and it is more than 10 times the amount accumulated by Oregon’s next-richest person, the Columbia Sportswear CEO Timothy Boyle.

He is an outsized presence in the small, Pacific north-west state, where the company he founded is Portland’s second-largest private employer. He stepped away from day-to-day operations of the global sportswear giant in 2016 to become “chairman emeritus”.

The Nike founder, Phil Knight (center), during an NBA game between the Portland Trail Blazers and the Los Angeles Lakers. Photograph: Steve Dipaola/EPA

Knight is famously shy about talking to the media and has made no open or consistent policy demands. The closest he has come, as first reported in local publication Willamette Week, was telling a political consultant in an interview for a newsletter that he wanted the state’s pension system to be fixed. He followed this with nonspecific comments about Oregon’s dearth of political leadership.

“Phil Knight’s money always comes with strings attached,” said the author Joshua Hunt, who has written a new book, The University of Nike, about Nike’s relationship with the University of Oregon. “He uses philanthropy as a cudgel to get what he wants.”

The university is nominally a public institution but, Hunt argued, it has increasingly come under the control of Knight and the corporation he founded. Knight has donated almost $800m to the institution, financing stadiums and helping build the college’s football and track team into national powerhouses.

But in the process, Hunt said, he has bent a public institution to his will. Hunt points to meddling large and small – from Knight’s insistence that coaches and athletic directors be replaced to what Hunt alleges was a challenge to the university’s participation in workers rights initiatives at the turn of the century.

Hunt said: “Were he to get even more, it would not be used to make Oregon a better place for Oregonians. It would be used to make Oregon a better place for Phil Knight.”

The enormity of Knight’s donation has prompted a broader debate of Oregon law, under which there is currently no upper limit on what someone can spend in an election, said Patrick Starnes, who was standing a gubernatorial candidate from the Independent Party until he recently decided not to run.

“Campaign finance reform was not on the agenda two weeks ago,” he said. “Now it is.”