FULLERTON, Calif. — The Democrats’ message to Mai Khanh Tran was polite but unsparing. With half a dozen Democrats running for Congress in her Orange County district, they showed her a discouraging poll and argued that she could not win — and risked fracturing the party in the June primary election.

Ms. Tran pointedly replied that she was “the only qualified woman, the only immigrant and the only physician in the race.”

“I said to them, frankly, let the voters decide,” recalled Ms. Tran, a pediatrician.

The national Democratic Party was not chastened: On Wednesday, the Democratic Congressional Campaign Committee took sides in that House race and backed Gil Cisneros, a Navy veteran and former Republican.

With their forceful intervention in Orange County, national Democrats have lunged into an impatient new phase of the 2018 primary season — one in which they are clashing more openly with candidates and local political chieftains in their drive to assemble a slate of recruits for the midterms.

In districts from Southern California to Little Rock, Ark., and upstate New York, the party has begun interceding to help the Democrats it sees as best equipped to battle Republicans in the fall.

The approach is laced with peril for a party divided over matters of ideology and political strategy, and increasingly dominated by activists who tend to resent what they see as meddling from Washington. A Democratic effort to undercut a liberal insurgent in a Houston-area congressional primary in March stirred an outcry on the left and may have inadvertently helped drive support to that candidate, Laura Moser, who qualified for the runoff election next month.

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But in some areas, Democratic leaders have concluded it is worth enduring backlash to help a prized recruit or tame a chaotic primary field.

They are moving most aggressively in California, where the state’s nonpartisan primaries present a unique hazard: State law requires all candidates to compete in the same preliminary election, with the top two finishers advancing to November. In a crowded field, if Democrats spread their votes across too many candidates, two Republicans could come out on top and advance together to the general election.

There are at least four races in California where Democrats fear such a lockout, including the 39th Congressional District, where in addition to Mr. Cisneros and Ms. Tran there are two other Democrats running: Sam Jammal, a youthful former congressional aide, and Andy Thorburn, a wealthy health insurance executive who is backed by allies of Senator Bernie Sanders, independent of Vermont. The district is among the most coveted for Democrats nationwide — a seat left open by the retirement of Representative Ed Royce, a popular Republican, in an area Hillary Clinton won by about 8 percentage points.

Ms. Tran disagreed with national Democrats picking favorites in the party’s House primary. “Let the voters decide,” she told them.CreditRozette Rago for The New York Times

National Democrats may also intervene in the Southern California districts held by Representatives Dana Rohrabacher and Jeff Denham, where multiple Republicans and Democrats are running, and in the seat held by Representative Darrell Issa, a Republican who is retiring. Voters receive mail-in ballots starting in early May, making the next few weeks exceptionally important.

House Majority PAC, a heavily financed Democratic group that spends millions in congressional elections, recently polled all four races and has been conducting digital surveys that simulate the complicated California ballot, according to people briefed on the group’s strategy. The super PAC has run ads in California in the past when Democrats have faced disaster in primary season.

Representative Judy Chu, a Los Angeles-area Democrat, said the open primaries had led Democrats to take unusual steps to prevent Republicans from dominating the first round of voting.

“That would stop our goal of taking the House back,” Ms. Chu said. “We have to have a viable candidate, and I think that if it does turn out to be a Democrat versus a Republican, the Democrat will win.”

Ms. Chu said the campaign committee’s endorsement of Mr. Cisneros was a signal to donors and volunteers that it was time to close ranks.

But picking favorites is not easy for Democrats: Until mid-March, Southern California lawmakers were divided in the 39th District race between Mr. Cisneros, who is backed by Representative Linda T. Sánchez, an influential member of the Democratic leadership team, and Jay Chen, another Democrat who was endorsed by Ms. Chu. It was only after Mr. Chen opted against running, with a call for party unity, that Ms. Chu and other Democrats swung behind Mr. Cisneros.

Ms. Sánchez said the glut of Democratic candidates remained concerning across California, and acknowledged having lobbied the Democratic Congressional Campaign Committee to back Mr. Cisneros. National leaders, Ms. Sánchez said, had “a role to play in terms of trying to talk to nonviable candidates and urging them to be team players.”

Meredith Kelly, a spokeswoman for the committee, said the group was taking action in California because voters “deserve to have a Democrat on the ballot in November.”

“Any decision to get involved in these races is toward that goal and based on intelligence from the ground in California, extensive data and partnerships with as many local allies as possible,” Ms. Kelly said.

In the 39th District, Democrats went beyond prodding underdogs like Ms. Tran, 52, to stand down. Mr. Thorburn said the D.C.C.C. presented him with polling that suggested attacks on his finances and business record would be damaging in the general election — data Mr. Thorburn dismissed out of hand. He said the committee clearly indicated its preference for Mr. Cisneros.

Mr. Thorburn, 74, is now the most unsettling rival for Mr. Cisneros and national Democrats, pairing a pointed ideological message with a personal fortune to spend on advertising. Deriding Mr. Cisneros as a “wishy-washy” newcomer to the party, Mr. Thorburn said he would strike back hard if the committee were to attack him, as it did Ms. Moser.

“I’m much more of a fighter than the national party,” Mr. Thorburn said, warning: “If they do something like they did in Texas, we would come back guns blazing.”

Mr. Cisneros has won over important state groups, including the muscular California Labor Federation. But his campaign office, at a strip mall in Brea, about 30 miles southeast of Los Angeles, showcases his national allies: One wall boasts an enormous sign from the gun-control group Giffords, which supports him, while another displays photographs of Mr. Cisneros with Barack and Michelle Obama.


Gil Cisneros, a Navy veteran and lottery winner who earned the backing of the Democratic Congressional Campaign Committee on Wednesday over rival Democrats.CreditRozette Rago for The New York Times

Mr. Cisneros, a soft-spoken 47-year-old who became a philanthropist and Democratic donor after winning the California lottery in 2010, said his unlikely biography was at the core of his message, along with issues like gun violence and health care. He said his views on those subjects had been consistent, though his party registration had changed. “From the beginning,” Mr. Cisneros said, “it’s always been about getting my story out.”

Interviews with voters in the Republican-held district revealed why the race is so promising and tumultuous for Democrats. A labyrinth of highways and shopping centers and residential developments, the district has shifted in recent years as its Latino and Asian-American communities have grown. But a dozen voters there said they were not following the race or remained undecided.

At a shopping mall in Fullerton, a hospital worker named Lynn — who declined to give her surname because she did not want to be identified at work — said she wanted to repudiate Republicans but had not picked an alternative. An Asian-American woman in her 40s, she said she was alarmed by President Trump’s environmental policies and what she described as an outpouring of overt bigotry.

“It’s not because I want a Democrat — I just don’t want a Republican,” she said. “Ever since you-know-who became president, people are really racist and it’s really, really obvious.”

Some voters sounded unlikely to take their cues from national parties. Outside a Fullerton coffee shop where Mr. Jammal was greeting voters, Adam De Leon said he was suspicious of the candidates using personal wealth to sway the race. Mr. De Leon, 72, said he favored Mr. Jammal, 36, because of his government experience.

“What does it tell you when people spend millions of dollars to get into a position that pays maybe $140,000 a year?” Mr. De Leon said, somewhat underestimating the $174,000 congressional salary. “It’s all about power and connections.”

The Republican field is in flux, too. Young Kim, a longtime aide to Mr. Royce, is the front-runner but has several candidates challenging her from the right. With Republicans in Washington focused on defending beleaguered incumbents, they have been less intent than Democrats on shaping open primaries.

For Democrats, that project extends beyond California: On the same day the D.C.C.C. endorsed Mr. Cisneros, it also boosted candidates in New York and Arkansas who face contested primaries. In New York, the committee enlisted Juanita Perez Williams, a former candidate for mayor of Syracuse, to challenge Representative John Katko this month, though a lower-profile Democrat was already running with the support of local party leaders.

That kind of big-footing may be trickier in California. Mr. Chen, the Democrat who opted out of the 39th District race, said the party still faces a “precarious situation” there. He said he had decided against running after conducting a poll that showed him neck and neck with Mr. Cisneros and Mr. Thorburn — but with Democratic voters fragmented enough to create an all-Republican general election.

He predicted none of the remaining Democrats would follow his lead and get out.

“If you’ve never been involved in the party before and you just ran because you want to run, then you don’t really have those considerations,” Mr. Chen said. “They are new to this. They don’t have bridges to burn.”