Internal pressures oblige presidents of Mexico to spit in the eye of Uncle Sam. So the friendliness symbolized by President-elect Reagan's visit to President Lopez Portillo in Juarez on Monday is not to be dismissed.

Within the context of more personal harmony at the top, there is a possibility for a distinct improvement in the substance for an accommodating American stand on the issues of immigration, energy and trade. Reagan can fairly ask Lopez Portillo for a discret lowering of Mexican support for radical forces now surging in Central America.

By tradition, mexico is a revolutionary country. Its history features revolts against Spain, France, the Church and Yankee imperialism. Such wreckers of order as Benito Juarez, Emiliano Zapata and Pancho Villa fill the national pantheon. Main streets in the capital bear names like Avenida del Revolucion and Avenida de los Insurgentes. The ruling political force -- the source of all power in the country indeed -- is the Institutional Revolutionary Party (PRI).

In fact, Mexico is a land marked by dramatic inequality. About 10 percent of the population earns about 40 percent of the income. The booming mills of Monterrey and the gleaming skyscrapers of Mexico City abut abysmal slums that are themselves huge improvements over horrendous rural poverty. The PRI sustains all these manifold injustices by control -- thanks to not a little bribery -- of the presidency, the congress, the regional governorships and the chief organizations for workers and peasants.

The tension between tradition and reality is squared by a systematically anti-American ideology. The evils of American business, American militarism and American culture not only are sedulously propagated by the Mexican left, but are the coin of the realm -- a set of beliefs common to leading figures in politics, business, the universities and the media.

LP, as Lopez Portillo is known familiarly, has been more prone to those views than most of his predecessors. He has repeatedly had rough words for President Carter personally. He has driven hard bargains on sales of oil and gas, and has been sticky about arrangements for immigration. He has demanded access to American markets without freeing Mexican restrictions on trade.

On op of that, these past four years have witnessed a change by the two countries in their approach to hemispheric affairs. Prior to Carter and LP, the United States was notorious for sustaining corrupt, right-wing governments, while Mexico took a position of political neutrality.

Carter tried to force some more progressive regimes in the Caribbean and Central America. With the United States moving leftward, LP displaced Mexico in the same direction. He has been a warm backer of Fidel Castro and sponsor of left-wing movements in Nicaragua, Guatemala and El Salvador. a

Reagan clearly wants to get it back together with Mexico. His heavy courtship of the Hispanic vote in Texas and California is one gauge of that intent. Another -- though the wording was inept and lent itself to Mexican fears of an American energy grab -- was his proposal, on announcing his candidacy in November 1979, for a North American Accord with Canada and Mexico. Still a third is the meeting in Juarez with LP.

In moving toward harmony with Mexico, Reagan possesses some notable assets. As a friend of Big Oil, he is prepared -- as Jimmy Carter was not -- to have the United States pay top dollar for Mexican oil and gas. As a friend of the growers, and with slim ties to labor, Reagan can open the U.S. job market to Mexican workers. As a free marketeer, he opposes restrictions on Mexican exports.

Identifying the trade-off for that tilt toward Mexico comes easily. Reagan does not share the feeling of the Carter administration that amends have to be made to Fidel Castro. He does not wax lyrical about "progressive" regimes in Central America. If anything, he favors making life harder for Castro and his allies, while standing by this country's traditional friends.

Mexico, for its part, has no interest in promoting revolution in Central America. With the United States giving up promotion of "progressive" forces, the Mexican authorities no longer have to back those forces explicitly to prove they stand to the left of Washington.

So if Reagan makes the point subtly, if he concentrates on substance rather than rhetoric, it is feasible that Mexico will revert to its traditional neutral stance in hemispheric affairs. Such a move, particularly if carried over to the rule of LP's successor, would serve the interest of both the United States and Mexico in a more stable neighborhood.