Geoff Diehl, the Republican candidate for U.S. Senate, has long touted himself as the Massachusetts co-chairman of Donald Trump’s 2016 presidential campaign. He served as a convention delegate, volunteer and was an avid supporter of Trump.

But Diehl’s co-chair position with the campaign was honorary, with no independent management authority, and he didn’t have any officially sanctioned role after the March 2016 primary was over, according to people who worked with the Trump campaign in New York.

Since early 2016, Diehl has been widely known for his support of the president’s election efforts in Massachusetts. During his current race against incumbent Democratic Sen. Elizabeth Warren, media reports regularly refer to Diehl as the state campaign co-chair; the title is used to introduce Diehl in television interviews; and he has made mention of it on national television. Diehl also includes it on his campaign’s website.

However, an examination of Diehl’s role as co-chair revealed frustration among some Republicans, who believe his continued use of the title implies he played a more central role in Trump’s campaign than he actually did.

“It’s not false, but it’s also misleading,” said a senior Trump campaign source, who requested anonymity to speak about the on-going campaign of a fellow Republican.

Diehl campaign spokeswoman Holly Robichaud rejected that charge in a statement to WGBH News, saying questions about his co-chairmanship are politically motivated.

“This issue was brought up before the Republican primary and everyone knew it was laughable,” she said. “Clearly this is a last-minute desperate attack by those who did not support Geoff in the primary. Geoff’s endorsements by Vice President Mike Pence, former Speaker Newt Gingrich, and Sean Spicer to name a few speaks volumes.”

Diehl formally endorsed Trump on February 23, 2016, ahead of the March Massachusetts primary. An announcement that he would be named “state co-chair” was made by then-Trump state director Dean Cavaretta who highlighted Diehl’s new role with the campaign in a Facebook post.

The post noted the announcement was to be made by conservative talk show host Howie Carr later that day.

Cavaretta told WGBH News he was hired under contract by Donald J. Trump Inc. for the duration of the New Hampshire and Massachusetts primaries. He said the terms of his employment with the campaign gave him “broad discretion on organizing the state,” which included appointing Diehl and Charlie Malo, as “honorary co-chairs.” Malo declined to comment for this story.

“I never got any push back” from New York, Cavaretta said. “I never received any communications saying don’t do this.”

Diehl’s status as a state legislator, Cavaretta said, was an asset to the campaign. Cavaretta said he would often use Diehl at campaign events and as a media spokesman, with approval by then-campaign manager Corey Lewandowski and communications chief Hope Hicks.

“There was a series of emails between Corey and Hope and Dean and myself … just running it by them that it would be alright for me to be the co-chair,” Diehl told the Worcester Telegram & Gazette in a video posted to Facebook last week. “So, I became the co-chair of the campaign and that was just the title I was given and retained throughout the campaign.”

Later in the interview Diehl added: “There was never a time when I wasn’t representing the campaign in Massachusetts.”

In an earlier interview with WGBH News, Cavaretta gave a different description of Diehl’s role after Trump’s Massachusetts primary victory, as efforts turned to rounding up delegates for the eventual nominee.

“We were technically all volunteers” at that point, he said of April caucuses held in each of the state’s nine congressional districts.

Cavaretta noted that, by that time, campaign related matters “went through Vincent,” a reference to then-Massachusetts campaign chairman, Vincent DeVito, who did not comment for this story.

Diehl did aid the campaign during the Massachusetts caucuses — including making an appearance in a video aimed at rallying delegates. In the video, Diehl references his title as co-chair.

Diehl and Cavaretta were ultimately selected as part of the slate of 42 delegates from Massachusetts, 22 of whom supported Trump at the Republican National Convention later that summer.

At the time, Paul Manafort served as the campaign convention manager. In May 2016, Manafort was promoted to chairman of the national campaign.

According to a Trump campaign source, Diehl was not an official part of the campaign during the caucuses or during the convention.

Diehl did, however, act as a media spokesman for the Massachusetts delegation during the convention and conducted interviews with local outlets in which he was introduced or was noted as the campaign’s Massachusetts co-chair.

“Bay Staters from all backgrounds and communities came together to support Donald Trump. That included Geoff Diehl,” said Trump supporter and current GOP State Committeewoman Amy Carnevale, who also served as a convention delegate.

But pressed on Diehl’s status as co-chair during the campaign, Carnevale said she could not recall him holding any authorized position.

Jeanne Kangas, vice chair of the Massachusetts Republican Party and Trump supporter, said in an interview she could not say whether Diehl had an official role: “So be it. I can’t say he wasn’t, but I sure can’t say he was.”

Both Kangas and Carnevale told WGBH News they support and plan to vote for Diehl.

The lack of an organized structure and on-the-ground operation was a common criticism of the national Trump campaign in 2016. Between the Massachusetts primary and Trump’s election night victory, the national campaign cycled through multiple campaign managers and staff — a point made by multiple people interviewed for this story.

Diehl’s title as co-chair and even his support for Trump has been challenged on social media by some conservatives, including by independent U.S. Senate candidate Shiva Ayyudurai.

But Diehl has remained a loyal supporter of the president since the beginning. Ginny Greiman, a fellow Massachusetts delegate to the 2016 convention told WGBH News that “[Diehl] was out there all the time when other people were shrinking violets. I think there were people who wanted to make it seem like Geoff didn’t support Trump, but I never saw any indication of that.”

Pence waded into the Senate race this month with a fundraising email in support of Diehl and his challenge against Warren. The email touted Diehl as a “rock-solid conservative with a tremendous record” and lauded him for saving taxpayers billions, a reference to Diehl’s work in defeating an automatic increase in the gas tax. The email made no mention of Diehl’s involvement in the 2016 Trump campaign.

Despite Warren’s status as one of the president’s top Democratic targets and potential 2020 rival, Trump has not yet offered a public endorsement of Diehl, though according to the election-tracing website Ballotpedia, the president has endorsed 21 other Senate candidates.