ActBlue, the online fund-raising organization that powers Democratic candidates, has plunged into turmoil, with at least seven senior officials resigning late last month and a remaining lawyer suggesting he faced internal retaliation.

The departures from ActBlue, which helps raise money for Democrats running for office at all levels of government, come as the group is under investigation by congressional Republicans. They have advanced legislation that some Democrats warn could be used to debilitate what is the party’s leading fund-raising operation.

The exodus has set off deep concerns about ActBlue’s future. Last week, two unions representing the group’s workers sent a blistering letter to ActBlue’s board of directors that listed the seven officials who had left. The letter described an “alarming pattern” of departures that was “eroding our confidence in the stability of the organization.”

What prompted so many longtime ActBlue officials to leave is not clear — none of the former officials agreed to be interviewed on the record.

“Like many organizations, as we undergo some transition heading into this new election cycle, we are focused on ensuring we have a strong team in place,” said Megan Hughes, an ActBlue spokeswoman. “We greatly appreciate the contributions of our incredible team members and remain deeply committed to the success of our organization and our mission to enable grass-roots supporters to make their voices heard.”

According to the letter from the ActBlue unions, which has not been previously reported and was confirmed as authentic by three people briefed on its contents, the senior staff departures began on Feb. 21. That day, ActBlue’s customer service and partnerships directors, who had both worked at the group for more than a decade, left, according to the unions’ letter.