WASHINGTON — A powerful but little-known group of Republican donors installed by President Trump to oversee the United States Postal Service has helped raise more than $3 million to support him and hundreds of millions more for his party over the past decade, prompting concerns about partisan bias at the agency before the November election.The largest amount of fund-raising has been by groups with connections to Robert M. Duncan, who continues to sit on the boards of two super PACs pushing for Republicans to win in 2020, one of which has spent more than $1 million supporting the president’s re-election. But he is only one of five Republican members Mr. Trump has named to the board — most of whom have given generously to the party — who have taken a hands-on role in trying to defend the embattled agency against accusations that it is trying to help the president win a second term by sabotaging voting by mail.At least one of the governors expressed concerns in an interview like those voiced by the president about possible voter fraud, citing an anonymously sourced news report circulated by the Trump campaign and the president’s son Eric Trump about how mail-in ballots can be manipulated.“If any doubt is ever raised — like in the New York Post article, or by any other reputable publication — we want to get to the bottom of that,” said John M. Barger, one of the Republican board members named by Mr. Trump and a participant in a newly formed election mail task force.Other governors have done little to hide their loyalty to the president, even as the board meets behind closed doors to plot a strategy for handling what is expected to be a record crush of mail-in ballots this fall.Hours after Mr. Duncan assured lawmakers at a Capitol Hill hearing last month that he was committed to doing his job without partisan bias and according to “the public interest,” he appeared on video at the Republican National Convention, holding up four fingers and smiling as fellow Kentuckians chanted, “Four more years!”For Democrats who are increasingly concerned that Mr. Trump is bent on kneecapping the mail system to bolster his own re-election chances, the juxtaposition was an alarming reminder that the president has stacked the Postal Service board with allies who support him, and who can amplify and act on the concerns he has tried to sow about mail-in voting.A Postal Service spokesman did not provide specific responses to multiple questions concerning the board of governors, except to say that Mr. Duncan’s appearance at the Republican National Convention did not involve government resources and therefore did not violate federal law against partisan activities by federal workers. Mr. Duncan did not respond to requests for comment on Monday.The concerns about a partisan bent at the Postal Service were underscored this weekend when Mr. DeJoy was accused of cultivating an environment at his former company, New Breed Logistics, that left employees feeling pressured to donate to Republican candidates, and rewarded them with bonuses for doing so.Senator Chuck Schumer, Democrat of New York and the minority leader, and Representative Carolyn B. Maloney, Democrat of New York and the chairwoman of the House Oversight and Reform Committee, called on the Postal Services’s board on Monday to suspend Mr. DeJoy while Ms. Maloney investigates the matter.Now members of the board are toiling to respond to new questions about the Postal Service without wading into a political quagmire created by the president who installed them, and the homes of Mr. DeJoy and the once low-profile board members have become the sites of protests.The agency has begun an advertising campaign, including a television spot, intended to assuage concerns about its ability to process the anticipated surge in voting by mail. But some officials there have cautioned against appearing to encourage the practice, which Mr. Trump has called fraudulent.As part of the initiative, the agency is likely to distribute a mailer to households this month to reassure voters that the Postal Service can handle the flood of mail-in ballots and to urge those who choose to cast them to send them early. But the mailer is expected to stop short of explicitly promoting voting by mail.“We know our lane,” Mr. Barger said in the interview. “And that is to let the American people know that the Postal Service will serve this election like it has all others. We are ready.”But Democrats worry that under Mr. Trump, the body has become too politicized to fulfill that role.“It’s appalling,” said Representative Peter Welch, Democrat of Vermont who has highlighted examples of partisan activity by top postal officials, including $3.2 million in Republican campaign contributions by Mr. DeJoy. “The Postal Service is respected and revered because it has a single job: delivering the mail, not serving the partisan interest of whoever happens to be president at the moment.The concerns among Democrats and government watchdog groups go beyond Mr. DeJoy. Five of the board’s seven members have a history of donating to Republicans, though some also have made more limited donations to Democrats. Mr. Duncan is a director for American Crossroads, which has spent more than $1.4 million since April on text messages supporting Mr. Trump’s re-election effort. He is also a director of the Senate Leadership Fund, a super PAC linked to Senator Mitch McConnell of Kentucky, the majority leader and one of his close allies, that has raised about $129 million. Since 2016, Mr. DeJoy has contributed more than $1.2 million in support of Mr. Trump’s campaigns.With Mr. DeJoy and Mr. Duncan, the three other Republican governors — Mr. Barger, Roman Martinez IV and William D. Zollars — make up a majority of the board, while only two members are Democrats. Steven Mnuchin, the Treasury secretary, helped vet prospective nominees to the board for Mr. Trump, recommending candidates who would carry out changes that the administration sought. Mr. Mnuchin’s recommendation of Mr. Barger, a California lawyer and financial investment adviser, was seen as helpful, because while he had donated more than $90,000 to Republicans since 2010, he had never given to Mr. Trump.The board tapped Mr. DeJoy to become postmaster general in June, and he has overseen the removal of hundreds of mail-sorting machines and the limiting of overtime, moves that have coincided with a well-documented slowdown in mail delivery.The Postal Service board has long been a landing pad for presidential loyalists of both parties, many of whom have continued to donate prolifically to candidates during their tenures. President Bill Clinton nominated a former member of his transition team who served as national finance chair of Hillary Clinton’s 2008 presidential campaign while serving on the board. But Mr. Trump has been able to shape the Postal Service more than most presidents, entering the White House when not a single governor was in place — Republicans had blocked all of President Barack Obama’s nominees — and as its long-term fiscal viability was increasingly in doubt.“It’s pretty ominous,” Mr. Welch said. “It really is different. It’s not the way it’s always been.”Mr. Trump’s skepticism about the quasi-public agency predates his presidency, but during his time in office, he has repeatedly bashed the agency, claiming that it essentially subsidized Amazon, another target of his ire. That company’s founder and chief executive, Jeff Bezos, owns The Washington Post, whose coverage has often angered Mr. Trump.The board members appointed by Mr. Trump have mostly stood by Mr. DeJoy as the changes he has overseen have drawn mounting criticism, though one Democratic governor told lawmakers last month that he had resigned in protest, in part because of fears about the politicization of the agency. The remaining board members have convened more frequently — albeit by teleconference — during the pandemic, with governors seeking greater oversight into the day-to-day operations of the Postal Service heading into the election. The full board is scheduled to hold an in-person meeting on Wednesday morning in Washington. And the bipartisan Expanded Election Mail Task Force met for the first time on Thursday.“The meeting reaffirmed my faith that the Postal Service is fully ready, willing and committed to deliver the nation’s election mail timely and securely,” Mr. DeJoy said in a statement.In the interview, Mr. Barger said he did not recall any formal discussions among the board of governors about vote-by-mail fraud concerns. But he said he was personally worried about an article in the New York Post last month that quoted a single unnamed Democratic operative claiming without proof that mail carriers have helped him commit voter fraud in past elections.“There is a high level trust that I have, that the board has, and that the American people have for the men and women who work for the Postal Service, and we want to protect that,” Mr. Barger said.Democrats are demanding greater transparency and strict restrictions on political activity by the governors. Senator Elizabeth Warren, Democrat of Massachusetts, has called on them to turn over their financial disclosure forms to Congress, so lawmakers can determine whether they have conflicts of interest in business dealings that come before the Postal Service. So far, the agency has declined to release the documents.Ms. Maloney has introduced legislation that would bar board members from participating in partisan activities — as Mr. Duncan does through the super PACs he helps lead — and disallow the hiring of a postmaster general who has made political campaign donations within four years of being appointed, as Mr. DeJoy has done.“By law, the Postal Service is supposed to be independent and nonpartisan, but President Trump has turned the law on its head,” Ms. Maloney said in an email.Like Mr. DeJoy, other board members are facing scrutiny for potential conflicts. Mr. Zollars, the former chief executive of YRC Worldwide, the beleaguered trucking company that received a $700 million loan from the Treasury Department in exchange for a 30 percent stake, was confirmed by the Senate in June. Mr. Mnuchin intervened to steer the loan to YRC, which has ties to Mr. Trump and his family.Mr. Zollars also sits on the board of Prologis, a logistics real estate company and Postal Service contractor, and has a financial stake in the business, according to recent financial disclosures.Stuart Gilman, a former top official at the Office of Government Ethics, said the holdings raised questions of a conflict of interest.“You’re responsible for avoiding even the appearance of conflict of interest,” he said. “How and in what way does the ethics official justify these sorts of holdings?”Under sharp questioning from congressional Democrats at the hearing last month, Mr. Duncan distanced himself from a 2008 fund-raising mailer he signed when he oversaw the Republican National Committee, which asserted that “Obama-Biden Democrats” were trying to “steal” elections from Republicans.“I don’t believe anyone at this point who is a nominee of the major parties is trying to steal an election,” Mr. Duncan said.Nevertheless, Representative Ayanna Pressley, Democrat of Massachusetts, said she believed he should step down from his role at American Crossroads while at the helm of the Postal Service board.“The U.S.P.S. deserves leadership that will put the public interest ahead of politics,” she said.