The "president-designate" of Mexico, budget boss Carlos Salinas de Gortari, will be as reasonable in his dealings with the United States as his mentor, President Miguel de la Madrid, has been, our sources in Mexico say.

But the Central Intelligence Agency isn't so sure about Salinas. The agency is afraid the future president is a leftist, who will not only promote socialist economic policies but who will prove to be an obstreperous and troublesome opponent of Washington's foreign policy objectives, particularly in Central America.

Salinas' selection as the candidate of the ruling Institutional Revolutionary Party, known as the PRI, which hasn't lost a national election in 58 years, followed the outward pattern of such designations, but with a significant difference. In the past, the imperial Mexican presidents have chosen their successors in strict secrecy, and without consulting party leaders.

De la Madrid confided in a private interview earlier this year with Dale Van Atta that he would make his decision on a successor only after extensive consultation with PRI leaders. He went further than that, in fact: He promised that the list of possible candidates "will be published when the time comes," which was in August.

De la Madrid declined to say which of the six possible candidates he was leaning toward, and his choice surprised many Mexican political observers. But we had received strong hints from various sources that he would settle on the uncharismatic, Harvard-trained economist, and his decision makes a lot of sense.

Salinas was the gray eminence of de la Madrid's Cabinet, the behind-the-scenes strategist who devised and implemented much of the president's economic reform program. Choosing Salinas as his successor was proof that de la Madrid understands the seriousness of Mexico's economic situation, and intends to make sure that the reforms will continue after he leaves office next year.

Sources in Mexico City tell us that Salinas was the chief architect of what might be called Mexico's attempt at a Reagan-Thatcher type of "privatization": selling off money-losing public corporations and buoying the private sector. Part of the policy is to eliminate protectionist policies that Salinas believes are counterproductive and inflationary.

Our evaluation of Salinas, based on conversations with him and sources in Mexico, is that he is a capitalist in a thin socialist disguise.

The CIA disagrees. A secret profile we obtained has this to say:

"Salinas is an ambitious technocrat and an expert economist. He has been a protege of de la Madrid's since the mid-1970s . . . . He has good family connections, he became active in the PRI early on, and he has excellent academic credentials.

"When he first emerged as a force to be reckoned with in early 1984, most local observers saw him as favoring business and free enterprise, but more recent reporting from reliable sources indicates that he belongs to the left wing of the PRI, and he may favor greater state domination of corporations that are now owned mostly by U.S. and multinational firms."