In Mexico, as in the United States, this is the season of the presidential campaign.

Politicians' faces adorn posters and billboards, airwaves and newspaper pages are full of election stories, and the opposition is hammering away at government policies on inflation and unemployment.In certain aspects of style and substance, this year's Mexican campaign resembles the contest taking place north of the border. But in other respects it is quite different.

Timing is one simple difference. Mexico's elections will take place July 6, before the Democrats and Republicans in the United States have even held their national conventions.

But a more important distinction is that in the United States, the two major parties are relatively evenly matched, while in Mexico one party has an overwhelming advantage.

Although opposition parties have existed from the beginning, the Institutional Revolutionary Party, or PRI, has not lost a presidential election since it was created in 1929.

Its structure, which formally incorporates nearly all of the country's organized-worker, peasant and middle-class organizations into the party, has enabled it to maintain tight control over the government and the electoral process.

This naturally has affected the level of suspense in the campaign. The question is not whether the PRI candidate, Carlos Salinas de Gortari, will be chosen as Mexico's president. That was effectively decided last fall, when he was selected as the party's standard-bearer.

Now that the campaign is on, the remaining questions are just how big a margin Salinas will win by, who will come in second and how well he will do.

Unlike in the United States, where candidates must devote several months to state primaries and caucuses and national conventions just to win their party's nomination, the bulk of a Mexican campaign is spent on the final contest. Candidate selection is neither as elaborate nor as uniform a process as in the United States. Each party handles it pretty much any way it wants.

The PRI does not hold a presidential primary or national convention. The candidate is named by the incumbent president.

Juan Enriquez Cabot, the PRI's foreign press director, stressed that the selection process is not unilateral, that the president takes into account the recommendations of party leaders and interest groups. But political scientist Lorenzo Meyer wrote in the magazine "Nexos" that although this time the usually secret selection process was modified with the announcement of six unofficial "pre-candidates," who each presented proposals to party leaders in public meetings, the changes were only in form, not substance.

"One more time it was the president and only the president who decided who should be his successor in 1988," he wrote.

The rightist National Action Party, or PAN, used a convention to select Manuel Clouthier as its candidate over contender Jesus Gonzalez Schmal, said Hector Vera Arenas, the party's presidential campaign director.

And the Mexican Socialist Party selected its candidate, Heberto Castillo, in an open primary, in which anyone interested was free to vote.

Castillo has since withdrawn and thrown his support to the main leftist challenger, Cuauhtemoc Cardenas, who became the candidate of a coalition of three small parties in a process of consultation with party leaders, not unlike the system used by the PRI.

As in the United States, parties competing for the presidency in Mexico can receive a certain number of campaign dollars from the government. But this money is distributed according to the proportion of the vote the party received in the previous election, rather than on the matching-fund basis used in the United States.

The PAN does not think this is fair, so it does not accept any of the government money, said Vera.

Altruism is doubtless not the only reason for the PAN policy, however. Although Clouthier said his party will spend in its entire campaign what the PRI spends in just one day, the PAN, with support from the middle class and private business, is best able of all the opposition parties to raise funds on its own.

And the party gains considerable political credibility by forgoing the government money.

More important than the PRI's share of the official campaign money, its opponents say, is its access to government resources at every turn.

But Octavio Campos Ortiz, subdirector of information for the Interior Department, which oversees elections through the Federal Electoral Commission, said the law prohibits government agencies from lending their resources to campaigns, and the opposition has not proved its accusations that violations occur.

When government employees take part in PRI campaign events, he said, they do so not in their role as government workers but as members of their respective unions.

In the United States, candidates must file public reports of all the campaign contributions they receive. Mexican law does not require this, said Campos Ortiz. Parties must report only how they spend the portion of their campaign money that comes from the government.

Adriana J. Gomez, associate director of foreign press for the PRI, said the party does not publish figures for its overall campaign expenditures because they are next to impossible to calculate. Many of the expenditures come in the form of in-kind donations from various sources at local, state and national levels, she said. Pro-government peasant and labor organizations, as well as businesses, provide services ranging from taxis to hotel rooms to publicity.

In one day alone of a recent campaign trip to Hidalgo, the Salinas campaign provided bus and taxi transportation, meals and hotel rooms for more than 90 journalists and perhaps 100 staff people and invited guests.

In contrast, Cardenas' National Democratic Front runs its campaign using borrowed cars and volunteer workers. Press spokesman Carlos Edmundo Berumen said the cash outlays for the campaign amount to about $220 per day.

Access to media is an area of great disparity between the parties. By law, each party gets at least 15 minutes of free television time and 15 minutes of free radio time every month. Berumen said a party can buy more if it can afford to do so, but most don't have enough money. Vera said the PAN on some occasions has even been refused time when it offered to pay.

Berumen said the opposition parties are given their free time at odd hours, and the government-owned network, Imevision, gives all sorts of coverage to the PRI that it does not give the other parties.

And the single privately owned national network, Televisa, which has a virtual monopoly over commercial programming, is owned by an avowed PRI supporter who openly favors the ruling party.

The PAN is angry enough about the unequal Televisa coverage that it is boycotting the network's flagship news show, "24 Hours," and is broadcasting messages by satellite from the United States to those Mexican viewers who own dish antennas. Renting the satellite is much cheaper than buying time on Mexican television, PAN officials say.

Although television is increasingly important in Mexican campaigns, it does not yet play the pivotal role that it does in U.S. politics. Enriquez, who is familiar with both systems, said the Mexican political culture is still much more oriented to direct personal contact. People want to see and be close to the candidates.

So for six months or more, the contenders trek around the country, holding rallies and politicking in the whistle-stop style that characterized U.S. campaigns half a century ago.

One common measure of a Mexican candidate's strength is the size of crowd he can draw. This has led not only to highly unreliable crowd counts by party officials and local newspapers, but also to accusations that the PRI frequently trucks in its crowds to make a good showing and uses various methods to pressure the people into coming.

Enriquez said Salinas has put the word out this year that he wants only people who attend his rallies of their own free will. But the press spokesman would not go so far as to say that the candidate has been able to entirely eliminate the trucking-in practice.

Polls, long an important fixture of U.S. elections, historically have not played a major role in Mexican campaigns. But this year that seems to be changing. The past two months have seen a flurry of surveys commissioned by a variety of groups. This year Gallup, through a Mexican affiliate, has done pre-election surveys and plans to do exit polls as well, said Harvard political scientist Jorge Dominguez, who helped design the Gallup questions.

The validity of several of the surveys, including those by Gallup, has been questioned, though. Some critics have said the results merely reflect the outcomes desired by their sponsors. Others have suggested that even the results of well-intentioned surveys may not carry the same significance as they would in the United States because Mexicans, for various reasons, may not be willing to express their honest opinions to a poll taker.

One final aspect of U.S. elections that some say has made or broken candidates in the past - the presidential debate - does not exist in Mexico.

The PRI candidate never debates his opponents.

Vera said PAN has made numerous requests for public debates, including one in writing in advance of a campaign stop when Salinas and Clouthier were to be in a town on the same day. But he said PRI officials replied that the conditions were not right.

The reasons the PRI gives for not debating are not so different from those offered by U.S. candidates who enjoy comfortable leads.

Enriquez said Salinas has no reason to confront his opponents - and by doing so give them importance they don't deserve - if they lack a "serious" program to discuss. Like front-runners everywhere, Salinas refuses to concede that his opposition's proposals are "serious."

And, perhaps most importantly, Enriquez said that, as in the United States and the rest of the world, a front-runner does not debate his opponents unless the vote is likely to be close.

Despite increasing competition, the PRI's advantage is still sufficiently ample that few observers expect the party's candidate to share a stage with his opponents in public debates anytime soon.