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.