Key members of President Donald Trump’s political team are facing a backlash over their efforts to promote a new small-dollar digital fundraising website called WinRed, with critics of the site demanding to know who stands to profit from the millions of dollars in donations it gathers.

A growing push by Trump and his advisors to require rank-and-file Republicans to use the WinRed fundraising website prompted detractors to blast an email Tuesday to the leadership of the Republican National Committee. The email is anonymous, signed by “concerned current and former RNC members,” and says it is asking “questions that deserve an answer.”

The email arrived as the RNC began a summer retreat in Charlotte, North Carolina, this week, and senior party leaders are scheduled to meet behind closed doors with WinRed executives on Wednesday to discuss brewing concerns about the site.

A recipient provided ABC News a copy of the email, and two other recipients confirmed that it was sent widely among party luminaries. It raises an array of concerns about the WinRed site, including the risk that those who run the site could control the flow of dollars -- and use the site as a weapon against Republican detractors.

“It’s clear you have the power to kick people off the platform at your whim,” the email says. “What proof do we have that you won’t punish members of the Freedom Caucus or conservative primary challengers [by kicking them] off the platform.”

Supporters of WinRed attributed the email to a rival digital fundraising firm, Anedot, and said they intend to respond to all concerns at the private gathering of Republican leaders on Wednesday.

“WinRed is a revolutionary new tool that will transform the way GOP candidates and conservative causes across the country raise money. It has the full backing of President Trump and his campaign, and Republican campaigns and state parties across the country are already utilizing WinRed," Republican National Committee spokesperson Mike Reed said in a statement to ABC News. "It is disgusting that anonymous individuals are spreading lies about WinRed in an effort to protect their own bottom lines and continue the status quo of dysfunctional grassroots fundraising strategy."

"Conservative causes and campaigns are free to use whatever fundraising products they like," Reed added. "But if campaigns and causes are going to rely on RNC resources we will require the use of RNC supported technology.”

WinRed representatives did not immediately respond to ABC News' request for comment.

Trump announced his support for WinRed on Twitter last month, saying, “This new platform will allow my campaign and other Republicans to compete with the Democrats' money machine. This has been a priority of mine and I’m pleased to share that it is up and running!”

A Trump campaign official said that “no Trump campaign employee benefits financially from WinRed, which is an effective tool to aggregate fundraising for the GOP.”

Trump has shown political dominance over his party, placing loyalists in key positions around the country. But the dispute highlights a remaining quarter of resistance within the veteran political establishment to attempts by the president’s allies to assume power over the highly profitable industry that makes money by turning the gears of American politics.

Sources close to the Trump campaign and the White House have expressed concern about the opaque corporate and ownership hierarchy at WinRed and other key vendors to the campaign and the RNC. Some have questioned whether key decision makers at the campaign and the party have any financial connection to those vendors. WinRed was formed with the help of another firm, Data Trust. Trump campaign manager Brad Parscale sits on the DataTrust board of directors. An RNC official told ABC News that no one on the board of directors is benefiting from the contract.

“Individuals hiding in the shadows should come out in the interest of transparency,” one Republican National Committeeman told ABC News. “We don’t know who at WinRed, Revv, Data Trust and other groups involved is making all of the money.”

The RNC member spoke on the condition of anonymity, because of concern that detractors could face reprisal. The anonymous letter raises the same concern, stating, “Are any RNC staffers benefitting financially from the WinRed contract?”

Reed told ABC News that no Republican National Committee staffers were benefiting financially from the contract.

WinRed was presented as an answer to ActBlue, an online one-stop shop where Democratic donors can direct donations to the party’s array of candidates in races around the county. Unlike WinRed, ActBlue is a non-profit entity.

WinRed stands to generate millions in income, as it takes a percentage of every contribution. It would also become a repository of names and other details about Republican donors -- a valuable resource both politically and commercially.