Mexican voters go to the polls Sunday in an election that many Mexican analysts contend will test the credibility of their country's political system. Voters nationwide will elect a new lower house of Congress, and seven states will choose governors.

For the first time, government and opposition politicians report, the conservative National Action Party is mounting a serious challenge to Mexico's ruling Institutional Revolutionary Party in two gubernatorial races and in as many as 30 of the country's 300 congressional districts. The PRI, as the government party is known, feared until recently that National Action had its best chance of scoring a historic upset in the Sonora governorship contest. But some PRI strategists profess now to be more concerned about the state house race here in Nuevo Leon.

"Our Sonora problem is solved, but Nuevo Leon still looks difficult, though we will win there, too," one PRI official said. As the country's most industrialized and urbanized state and home to much of the opposition party's national leadership, Nuevo Leon is where the PRI should be most vulnerable to an opposition assault on its 56-year-old monopoly control of Mexico's state governments.

However, few independent analysts now expect National Action to win as many as half the votes either in Nuevo Leon or in Sonora. Published polls conducted here by state universities, market research companies and the Unified Socialist Party concur that while National Action strength is at a record high, it is still eclipsed by the PRI's declared support. EL Norte, a Monterrey daily newspaper sympathetic to the opposition, reported that its voter survey showed the PRI winning by nearly 2 to 1.

In Nuevo Leon, the campaign of National Action candidate Fernando Canales Clariond has benefited from the lackluster stump performance of his rival, the PRI's Jorge Trevino, an amiable one-term congressman far more comfortable at sketching out state economic development plans than at making speeches.

Both well-educated products of upper-class Monterrey business families, Trevino, 49, and Canales, 39, agree publicly on such issues as the need to stimulate more private investment in the greater Monterrey area, Mexico's largest manufacturing center, which accounts for about 70 percent of the state's voting population of 1.3 million. But they disagree vehemently on the performance of past PRI governents, which Trevino loyally defends and Canales attacks as corrupt, fiscally inept and "unwilling to surrender power."

Canales closed his campaign Wednesday with 10,000 supporters chanting his optimistic slogan: "Yes, it is possible" to defeat the PRI.

But ideological combat in this election, in Nuevo Leon and elsewhere, has been obscured by acrimonious debate about the electoral process itself. National Action, backed by the Unified Socialist Party and other leftist opposition groups, contends that the PRI plans to discourage opposition voting by such tactics as the last-minute disqualification of antigovernment poll watchers and the elimination of polling booths in areas of opposition strengths.

Moreover, they say, voting lists have been "inflated" with false and duplicate names. Opposition groups in Sonora, Chihuahua, Yucatan, Mexico City and other areas where the PRI has done poorly recently have presented evidence of altered lists to authorities. In Nuevo Leon, National Action State Chairman Luis J. Prieto complains, whole nonexistent streets are listed.

Independent observers say many of the voting register mistakes are simply clerical. The government earlier this week announced it has "purged" a half million suspect names from the lists, reducing them to 35.3 million of the country's 40 million citizens of voting age. Yet in many instances the registers clearly have been doctored for political ends, several politically unaffiliated academic analysts say. "Some are errors, but the others are horrors," said Juan Molinar, a political scientist who specializes in election analysis.

PRI leaders retort that these charges are an attempt by members of the opposition to blunt the impact of their inevitable defeat. "How can they talk about fraud when the election hasn't even been held yet?" Gov. Alfonso Martinez Dominguez asked angrily.

Many political observers believe the question of election management continues to be the National Action's most effective campaign issue. Skepticism nationally about the accuracy of official returns increased two years ago, when the manipulation of voting results to overturn probable National Action mayoral victories in the state capitals of Baja California and Puebla was documented convincingly in the local press.