United Farm Workers president Cesar Chavez accused the Carter administration yesterday of undermining the union's West Coast lettuce strike by not enforcing immigration laws against illegal aliens used as "strike breakers."

Chavez, who acknowledged that the strike is a make-or-break test for the union, made the charge at a meeting with reporters here during an East Coast trip to drum up support for a strike-related boycott.

Officials of the U.S. Immigration and Naturalization Service denied the Chavez charge, saying the government is "bending over backwards to maintain a neutral position" despite pressures from growers and union leaders alike to intervene.

"We'll try to check out all complaints, but we can't stop folks just because they look 'foreign'," said INS Commissioner Leonel J. Castillo. Chavez "wants us to support his strike," said Verne Jervis, INS public information officer. "He wants us to enforce the law when he's out on strike. The rest of the time he doesn't."

The UFW has been on strike against 11 growers in California's Imperial Valley since mid-January in a dispute over its demand for major wage and benefit increases, reportedly up to 200 percent in some cases.

Inconclusive so far, the strike is expected to enter a critical phase when the spring-summer harvest starts in two or three weeks in the Salinas Valley in central California.

One of the principal strike targets has been Sun Harvest Inc., a subsidiary of United Brands Co. and the nation's third-largest lettuce producer. The union's boycott is aimed at Chiquita brand bananas, the product of another United Brands subsidiary and an easy boycott target because each banan carries its own label.

With up to 30 percent of its 100,000 contract-covered members working in lettuce fields, the union deems the strike and boycott critical because it wants to prove itself as effective a labor organization as it is a social movement. "This is so serious that our whole union could be destroyed in the strike," Chavez said yesterday.

Calling the government's immigration enforcement efforts a "farce," Chavez said that, despite 200 complaints from UFW members about illegal aliens working as strike breaders, the INS will not go into struck ranches to search them out. He added that there are "some whole camps full of illegal aliens" in the Salinas Valley just waiting for the harvest to start.

"We get checked more [by INS] on the picket lines . . . the illegals feel safer in the fields," Chavez complained.

Castillo said he was "sympathetic with Cesar's frustration" but said many of the union's complaints have been vague or unproductive when checked out. Ranch searches have been avoided if they could contribute to already tense situations, he said.

INS officials said arrests of illegal aliens hit a record 52,000 in California last month, up 35 percent from a year ago. That may be true, said Chavez, but they were "not from the struck fields, that's for sure."

Chavez said he is seeking a "blue ribbon" panel of labor, civil rights and church leaders to look into the California situation, possibly leading to congressional hearings. He said he had no immediate plans to appeal to White House officials because "they don't really care what our problems are."

As for next year's Democratic presidential politics, he said UFW members are cool to President Carter and look favorably on either Sen. Edward M. Kennedy (Mass.) or California Gov. Edmund G. (Jerry) Brown Jr.