When Jordan Wood, a progressive running for an open congressional seat in Maine, woke up on Monday morning, his husband looked at him as if someone had just died.

No one had. But while he was sleeping, Mr. Wood had missed some jarring news: An official from the Democratic Congressional Campaign Committee had left a voice mail message with the heads-up that the group would be announcing within hours that it was endorsing Joe Baldacci, a state senator and one of his primary opponents, in the race for Maine’s Second Congressional District.

“I’m in total shock,” Mr. Wood, who has raised more money than Mr. Baldacci but is trailing in some public polls, said in an interview. “They were pretty clear with me that they had no plans to weigh in on this primary.”

Mr. Wood’s was one of eight primaries in critical and competitive districts across the country where House Democrats’ campaign arm announced this week that it was getting involved to boost a candidate as part of its “red to blue” program, a sign of the party’s growing confidence that it is on track to win control of the House in November. Four of them were contested primaries, in which the D.C.C.C. chose one Democratic candidate over others.

Such endorsements, which typically translate into major fund-raising boosts, are not atypical. But this year, they have bitterly divided Democrats, who are feuding openly over the choices in a sign of deeper rifts about the party’s tactics and future. Some Democrats, including prominent members of Congress, are accusing the campaign committee of being undemocratic in trying to influence the selection of candidates in contested primaries before voters have had their say.

The intervention is drawing unfavorable comparisons to the 2024 election cycle, when some Democrats felt the party had anointed Vice President Kamala Harris as the presidential nominee instead of allowing a competitive process to play out following President Joseph R. Biden Jr.’s abrupt and late exit from the race.

In this case, Democratic critics say that the D.C.C.C. backed establishment-aligned candidates who it believed would not rock the boat and would support the current crop of House Democratic leaders, potentially at the expense of winning critical seats.

“It’s an extreme disconnect between party insiders who think they know best for our communities and our communities themselves,” said Randy Villegas, a candidate in California’s 22nd Congressional District who has the endorsement of the Congressional Progressive Caucus and the Congressional Hispanic Caucus’s campaign arm.

The D.C.C.C. this week passed over Mr. Villegas and instead chose to elevate Jasmeet Bains, a doctor and assemblywoman.

“This is about party leadership and D.C. elites putting their thumb on the scale for who they know will bend the knee to party leadership and corporate interests,” he said.

The co-chairs of the Congressional Progressive Caucus, which also supports Mr. Villegas, issued a statement on Monday criticizing the decision by the D.C.C.C. to, in their words, “tip the scales” in the race, and saying it should have stayed neutral.

Representative Linda T. Sánchez, Democrat of California and chairwoman of BOLD PAC, the political arm of the Hispanic Caucus, also said she was disappointed that the party had passed over strong Latino candidates whom the group supports. They include Mr. Villegas and Carol Obando-Derstine, a candidate in Pennsylvania’s Seventh Congressional District, where the D.C.C.C. instead endorsed Bob Brooks, a retired firefighter.

“Latino voters and candidates — like Randy and Carol — are not a small factor in the fight for the House majority, they are central to it,” Ms. Sánchez said in a statement. “The Democratic Party has clearly made progress earning Latino trust, but we still have a long way to go.”

In Arizona, House Democrats’ campaign arm did endorse a Latina candidate, Marlene Galán-Woods, in a competitive Democratic primary to replace Representative David Schweikert, the Republican who is leaving Congress to run for governor.

The decision enraged Amish Shah, a doctor and former state legislator who is mounting his second run in Arizona’s First Congressional District, after having won the primary in 2024 and come within a few points of defeating Mr. Schweikert. That year, Ms. Galán-Woods had finished third in the primary.

“They have broken all the credibility and trust,” Mr. Shah, who was leading Ms. Galán-Woods by a three-to-one margin in the only public poll of the race, said of the Democrats’ House campaign arm.

Mr. Shah said he received a call late Sunday evening from Julie Merz, the executive director of the D.C.C.C., telling him that it would be endorsing his opponent because she was more electable.

That came as a shock after he said he had been sharing weekly finance reports and confidential campaign data with D.C.C.C. officials, believing that they would refrain from intervening in the primary.

“They lied,” he said. “What can we do?”

Mr. Shah said that on the call with Ms. Merz, he had asked to see the data and polling that showed Ms. Galán-Woods, a former Republican, better positioned to win. That request was denied, he said.

“She said to me, ‘We do this with humility,’” he recalled of his conversation with Ms. Merz. “I said, ‘No, this is hubris, not humility.’”

The experience has deeply embittered Mr. Shah, who called for Democrats to stop backing the party apparatus in hopes of helping the party win the House majority.

“You’re fueling the broken cabal — stop donating,” he said, calling for Representative Susan DelBene of Washington, co-chairwoman of the committee, to lose her job.

A spokeswoman defended the endorsements.

“The D.C.C.C. has always been clear that we only weigh in on primaries where we feel one candidate puts Democrats in the strongest position to win the general election,” said Courtney Rice, the spokeswoman. “The stakes in this election could not be higher and requires us, in every race, to put our best foot forward and the D.C.C.C. is ensuring we do just that.”

It is not the first time such tensions have flared in a midterm election cycle.

“I believe you make your wave,” former House Speaker Nancy Pelosi said in 2018, defending her decision to back some candidates running in competitive primaries. “This is a coldblooded, strategic, focused campaign to win the Congress for the American people. We don’t waste time. We don’t waste energy, we don’t waste resources.”

That year, Democrats swept back to power in the House, winning a net gain of 40 seats.

This year, the D.C.C.C. said its decisions were based on deep assessments that include looking at a candidate’s local, state and national support; his or her momentum on the ground; fund-raising metrics; and how broad a coalition he or she is able to build.

Many of the candidates who were spurned, however, said they did not believe that their opponents were chosen because they could win. Mr. Villegas said he was leading Ms. Baines by every metric. He noted that she had dodged debates and political forums and that he had all the central committee chairs endorsing his campaign.

Mr. Wood suggested that he was passed over because he would not say whether he would support Representative Hakeem Jeffries, Democrat of New York and the minority leader, for speaker if Democrats won the House.

“I did not ask their permission to run and they don’t control my campaign,” he said.

Mr. Wood, a former congressional staff member who is viewed in Maine as a Washington insider, said he believed that he might ultimately benefit from the D.C.C.C. endorsing his opponent at a time when Democrats are fed up with party bigwigs.

At the state Democratic convention last weekend, Mr. Wood said that activists were buzzing about how badly party leaders had misread the electorate when they backed Gov. Janet Mills in the Senate primary against Graham Platner, the populist oysterman who has energized voters across the state. Last week, Ms. Mills suspended her campaign, saying she did not have the resources to finish out the primary.

“For them to do that on a Monday morning right after that happened in the Senate race — do you guys read the room?” Mr. Wood said of the D.C.C.C. endorsements. “They are probably sinking Joe Baldacci’s campaign.”