Sen. Jesse Helms (R-N.C.) charged yesterday that Mexican President Miguel de la Madrid was fraudulently elected in 1982 and that his party falsified legislative election results last year by keeping two sets of election books, "one public, one private."

Helms said documents he obtained from secret sources in Mexico show de la Madrid received 39.8 percent of the vote in 1982, rather than the 71.3 percent he claimed, a difference of 7.4 million votes. The ruling Institutional Revolutionary Party (PRI) coalition received 48 percent of the Chamber of Deputies vote last year instead of the 71.1 percent announced, or 5.9 million fewer votes, Helms said.

The Mexican Embassy immediately said the charges were "groundless and most probably are intended to confuse public opinion."

In a statement, the State Department said, "We have no reason to believe such charges and have never seen any evidence to support them."

"I believe this is a scandal sufficient to impeach the legitimacy of the government," Helms said at a hearing of the Senate Foreign Relations subcommittee on the Western Hemisphere, which he chairs. He urged the Mexican government "to open up the election process to international review and inspection," and said Mexico "deserves no help from the international community until this is done."

Helms has held two previous hearings in which he accused Mexican officials of fraud, corruption and links to drug trafficking. His subcommittee has jurisdiction over U.S.-Mexican relations, and Helms has expressed concern that deteriorating economic and social conditions in Mexico could lead to widespread political unrest requiring U.S. troops to be deployed along the border.

Helms' latest charges come as Mexico is completing renegotiations to ease this year's interest payments on its $97 billion foreign debt. U.S. banks and officials are heavily involved in the talks, along with the International Monetary Fund and other multilateral lending organizations.

An aide to Helms said the secret documents, which were not made public, came from within de la Madrid's government and were written in the military chief of staff's office for circulation among top PRI officials.

Helms charged that the Mexican government keeps "a double set of books, one public, one private. One set of statistics is the Mexican election results released by the Mexican Federal Election Commission. The other set is the real election results received by the presidential chief of staff of the military. Needless to say, the second set is considered top secret in Mexico."

In a statement, the Mexican government said the presidential military chief of staff "does not take any part in the Mexican electoral process" and that votes are counted with representatives of all parties present. A Mexican official said the chief of staff office only arranges presidential trips and interviews and would have had no reason to prepare any document on election results.

White House officials said there would be no comment on Helms' charges, but intelligence sources said nothing similar to them had surfaced previously. An official of the Senate Select Committee on Intelligence said nothing resembling Helms' charges has come to committee members' attention.

No Mexican political parties have accused de la Madrid or the PRI of altering the vote count sufficiently to change a national election. They have charged instead that the PRI buys votes and has stolen provincial races.

Helms' figures indicated that under the allegedly secret vote tallies, both de la Madrid and the PRI won the elections anyway, but with pluralities rather than majorities. The military tally cited by Helms alleged that in 1982, de la Madrid's coalition received 9.26 million votes, 39.78 percent, the conservative opposition National Action Party (PAN) coalition got 8.1 million, 35.1 percent, and the leftist candidates' coalition received 4.7 million, 20 percent, with 5 percent of the ballots invalid.

Helms said the PRI "is to a great extent tied directly into the system of graft and corruption," and is maintained through "socialist methods of economic control to direct graft and corruption to the ruling circles." A more capitalistic system would enrich the Mexican people, render graft an ineffective control method, and the PRI could lose power, he said.

"If the situation in Mexico continues to be one of fraud, corruption and the strangling of democracy, then vast infusions of U.S. taxpayers' cash will only open up more opportunities for corruption and fraud . . . . If Mexico wants U.S. help, the Mexican people have no choice but to bring about fundamental political reform," Helms said.

William D. Rogers, a Washington attorney who served as assistant secretary of state for inter-American affairs in 1974 and 1975, testified at the hearing that Helms' charges will trigger anti-American demonstrations in Mexico similar to those after U.S. Customs Commissioner William von Raab testified at a Helms hearing last month that virtually all Mexican law enforcement officials are corrupt.

"I can't think of a worse place to thrash out a U.S. policy than this hearing room," Rogers said. "It seems to me an attack on the Mexican presidency and its honesty and candor with the Mexican people is bound to be counterproductive . . . . It drives all Mexican officials into intoxicating nationalistic resentment."