Mexicans know who their new leader will be, but can he lead?

Miguel de la Madrid Hurtado is scooting about Mexico these days in a determined scramble to make his name a household word.

He has no doubt he will be elected president of this unique one-party democracy, come July. In December, if all goes according to schedule, he will be inaugurated for a six-year term.

But now he is campaigning as if his political future depended on it. And in a way it does, for Mr. de la Madrid is a new breed of presidential candidate: He is more technocrat than politician.

He has to prove to Mexicans that he has political savvy, as well as modern entrepreneurial and economic skills.

So far, the consensus here is that his campaign has not really caught on. Cynical Mexicans are even more cynical than usual about Mr. de la Madrid.

That does not mean they will not vote for him. They will. As candidate of the Institutional Revolutionary Party (PRI), a political amalgam that has dominated Mexican politics for 50 years, his election is assured.

But his success as president of this complex nation depends in considerable measure on his political ability.

The big question is whether he has the finesse to control the PRI. The party is an umbrella for diverse political elements - labor, business, academics, professionals, blue collar, white collar, leftist, rightist, centrist.

Mr. de la Madrid's speeches this past week - before gatherings of business leaders, trade unionists, and local politicians - were full of revolutionary rhetoric, befitting a candidate who represents a party with ''revolutionary'' in its title.

But the reception all along the way was reserved.

In a way, Mr. de la Madrid has a challenge similar to that of Miguel Aleman, the first real political candidate to be tapped by the PRI - after two decades of generals, men clearly associated with the events of the 1910 social revolution from which modern Mexico is dated.

There were plenty of skeptics then who doubted that Mr. Aleman would be able to hold post-World War II Mexico together. But he did with style and success - launching 35 years of control by politicians who brought Mexico a long way from the approach of the military men. Indeed, Mr. Aleman reduced the military's influence in modern Mexican life.

Now it is Mr. de la Madrid's turn to make another significant change in Mexico - to lead this nation into the 1980s, a more complex time, in terms of technology, politics, and economics, than most Mexicans realize.

Mr. de la Madrid is aware of the challenges facing his nation and is disturbed by his inability to get his campaign going.

One of Mr. de la Madrid's problems is his contacts with the neighboring United States. He speaks English fluently and spent a year at Harvard University in advanced study. In the PRI, his language ability and contacts can be a liability. He is trying to play that background down, refusing to speak English in public and using an interpreter he does not need. His official biography says he studied abroad; it does not mention the US.

All this may sound a bit strained. But in a society that is xenophobic, it is important. Sources close to Mr. de la Madrid are doing what they can to talk up the candidate's life in Mexico.

Mr. de la Madrid also is being extremely careful in outlining his domestic programs and foreign policy. He is known to feel that Mexico's support for leftist insurgents in El Salvador ought to be toned down and perhaps even dropped. But those insurgents are revolutionaries in the way that the PRI's forebears were in Mexico. Therefore they are worthy of support, in the eyes of many Mexicans.

But not all Mexicans agree. Many businessman, much of the top military establishment, and even some PRI leaders worry about the spread of insurgency through Central America and into southern Mexico if Salvadoran guerrillas win in their struggle with the Salvadoran goverment.

However, this is not a politically popular view in Mexic. Mr. de la Madrid and others who lean in that direction skirt it as much as possible.

The next few weeks will be critical for Mr. de la Madrid, for he is to speak to labor and business groups in many parts of the country. He needs to demostrate more political finesse to make his campaign catch on.