State Representative Geoff Diehl is as hardcore a Donald Trump-supporting Republican as any political figure in the state. But a decade ago, Diehl, now a leading candidate for his party’s US Senate nomination, was voting true blue.

It all began in 1996, when Diehl first registered to vote as a Democrat. He was diligent in carrying out his civic duties, casting votes in some of the state’s most liberal brawls. He voted in the 2006 gubernatorial primary when Deval Patrick pulled off a sweeping victory over Attorney General Tom Reilly and wealthy education advocate Chris Gabrieli.

His chief political adviser, Holly Robichaud, said Diehl voted for the moderate Reilly. And in the general election, she said, he rejected the liberal Patrick and instead voted for the GOP nominee, Kerry Healey.


Two years later, he cast a vote for Joe Biden — over front-runners Barack Obama and Hillary Clinton — in the state’s Democratic presidential primary.


“To try to stop Clinton and Obama,’’ Robichaud said.

Of course, the chances of Biden stopping those rivals had evaporated a month earlier when the Delaware Democrat officially bowed out of the race after placing fifth in the Iowa caucuses. His name could not be removed from the Massachusetts ballot before the vote.

If he did cast a vote for the future vice president, Diehl would have been one of two votes for Biden in the town of Whitman’s Precinct 3, where Clinton trounced Obama, 368 to 193. John Edwards, who dropped out a week before the Massachusetts primary, came in third, ahead of Biden with 18 votes. Biden got 0.26 percent of the total statewide vote.

In the state primary election six months later, Diehl would have had a choice for the US Senate Democratic nomination: incumbent John Kerry seeking reelection or Gloucester attorney Ed O’Reilly.


“He doesn’t remember Kerry having a primary,’’ Robichaud said.

In the 2008 general election, Robichaud said Diehl rejected his party’s nominee, voting for GOP candidate Jeffrey Beatty over Kerry.

She said he then cast a vote in the general election for GOP presidential nominee John McCain over Obama.

Diehl, who co-chaired Donald Trump’s state campaign in the 2016 presidential race, is now a strong favorite to win next month’s Republican convention endorsement of his Senate campaign. His opponents include Beth Lindstrom, a longtime activist in state Republican Party politics, and wealthy businessman John Kingston, who used his funds to rally the GOP to reject Trump 2016. The winner will challenge incumbent Democrat Elizabeth Warren.

Knowing that Diehl’s little-known political history could potentially undercut his support among delegates to the state Republican convention next month, Robichaud did what all good Republicans do when having to explain such political heresies: She wrapped her candidate around Ronald Reagan, a famous onetime Democrat who is now a GOP icon.

“Like Ronald Reagan, Geoff grew up in a Democrat household and left when he realized the party no longer stood up for working people,’’ Robichaud said. “He is a regular person and small business owner.”

Frank Phillips can be reached at frank.phillips@globe.com.