President Reagan charged today that Soviet-sponsored narcotics trafficking and terrorism represent "the most insidious and dangerous threats" to the Western Hemisphere.

In responses to a Mexican news agency on the eve of a meeting with Mexican President Miguel de la Madrid, Reagan employed his harshest anti-Soviet rhetoric since the November summit with Soviet leader Mikhail Gorbachev.

"This hemisphere is truly the cradle of democracy," Reagan said. "Communism is an unwanted, foreign ideology. The Soviets realize that it will never be established by choice in this hemisphere, so they resort to subversion and support for terrorism. Their malevolent activities in this hemisphere affect our bilateral relations with them."

Reagan's comments, prepared by his staff in response to questions submitted by Noticias de Mexico, charged that "the hand of the Soviet Union and its Cuban surrogate" is supporting terrorism in Colombia, Ecuador and El Salvador and "is behind the Nicaraguan government's subversion of its democratic neighbors."

"And the link between the governments of such Soviet allies as Cuba and Nicaragua and international trafficking and terrorism is becoming increasingly clear," he said. "These twin evils, narcotics trafficking and terrorism, represent the most insidious and dangerous threats to the hemisphere today."

Reagan is scheduled to fly to the Mexican border city of Mexicali Friday and meet for the fourth time with de la Madrid. The two leaders have a wide-ranging agenda that includes economic issues and Central America and are to spend at least an hour in private conversation.

Two senior U.S. officials who briefed reporters here about the visit painted a bleak picture of narcotics trafficking in Mexico. Despite increased cooperation between U.S. and Mexican authorities, one of the officials said, "unfortunately, the trend is not good."

The official said that in Mexico, as in Colombia, drug traffickers are becoming so rich that they comprise a political force with links to terrorists. Ultimately, the official said, this may pose a greater threat to U.S. security than do economic conditions in Mexico.

The official said that de la Madrid and Mexican Attorney General Sergio Garcia Ramirez are "honest and upright" but that "corruption" in high places has impaired efforts to control drug traffic.

Responding to a question about Reagan's link of terrorism and drug smuggling, one U.S. officials said that, in the last year, Libyan involvement in the Western Hemisphere has increased, mostly through "increased aid to violent groups."

Much of the discussion Friday is expected to focus on Mexico's serious debt problem. Mexico is seeking U.S. help in obtaining $4 billion in new loans and service interest on its nearly $100 billion in foreign debts.

The New York Times reported today that Reagan will assure de la Madrid that these loans will be forthcoming. The U.S. officials who briefed reporters said that Reagan is unlikely Friday to make any new commitment to help Mexico but that such a declaration is unnecessary because the United States has worked with Mexico since 1982 to help it resolve its debt crisis.

The officials observed, however, that Mexican inflation remains out of control and noted that the United States and world banking authorities are seeking Mexican economic reforms.