A lawsuit by two U.S. oil companies against Venezuela's state petroleum monopoly has shaken its administration with charges of corruption and violation of antitrust laws and created a potentially serious political problem for the government of President Jaime Lusinchi.

The two U.S. firms, Coastal Corp. of Houston and Mobile Bay Refining Co. of Mobile, Ala., are seeking $450 million from the state firm Petroleos de Venezuela for what they charge were arbitrary reductions in their supplies of Venezuelan heavy crude oil. Mobile Bay has charged that Venezuelan officials engaged in a conspiracy to drive its refinery out of business.

The suit, filed in September in a Texas district court, also alleges that Venezuelan officials accepted kickbacks and bribes for allowing other U.S. companies to resell supplies of Venezuelan petroleum at a premium. Lawyers for the companies say they hope to prove that the Venezuelans violated U.S. laws against commercial bribery and racketeering as well as the Sherman and Clayton Antitrust acts.

Venezuelan officials have uniformly denied the charges and described the suit as an effort by unscrupulous businessmen to extract increased deliveries of Venezuelan oil for their refineries. However, sources close to the oil industry here say there is concern that the legal action could damage a crucial state industry and endanger Venezuela's marketing practices for oil in the United States.

"There could be something in this case, and if so, it could be healthy for the oil industry to have any corruption exposed," said Rafael Tudela, a congressman and independent oil broker serving on a congressional committee examining the charges. "But that doesn't mean the situation is good."

In fact, the legal charges have come at a particularly hard time for Lusinchi's Democratic Action party government. Since taking office last February, the social democratic administration has been forced to adopt unpopular economic austerity measures even as it seeks to resolve a series of scandals that have weakened public confidence in Venezuela's democratic political establishment.

The allegations by the U.S. oil companies were made as a Venezuelan court prepared to begin the trials of Petroleos de Venezuela officials accused in a major bribery scandal uncovered last year. Ten officials, popularly referred to as the "petrospies," have been accused of participating in a scheme to sell confidential pricing information sought by buyers of Venezuelan oil.

The new charges, a lawyer for the U.S. companies said, are related to the "petrospy" case and will reveal more extensive corruption in the state oil company than Venezuelan authorities have acknowledged. "The Venezuelans have a serious problem with corruption," said Thomas McDade, an attorney with the Houston firm of Fulbright and Jaworski, in a telephone interview. "We intend to make them face up to it."

The suit has provoked a dramatic display of Venezuela's sensitivity to its petroleum industry, which is responsible for more than 90 percent of the country's export earnings, and its position as Latin America's most affluent nation.

In addition to the stream of official statements denying the charges, businessmen and politicians of all stripes have leaped to the state oil company's defense and portrayed the suit as a challenge to national sovereignty. "With such a libel, coming out of a blatant attempt at arm-twisting, no agreement or honorable settlement is conceivable," editorialized the left-wing magazine Resumen in a recent issue.

Other Venezuelan officials and industry experts describe the legal battle as an example of the tensions created between some oil producers and their consumers by production cutbacks mandated by the Organization of Petroleum Exporting Countries since last year.

Following its production guidelines as an OPEC member, Venezuela sharply reduced crude oil deliveries last year to a number of its U.S. customers, including Coastal Corp., whose daily quota was trimmed from 30,000 barrels a day to 15,000 barrels a day, officials here say.

The reduction angered Coastal officials, who say they had invested millions of dollars in altering a refinery in Corpus Christi, Tex., to handle Venezuela's heavy crude oil. In addition, McDade said, the reduction doomed the operations of Mobile Bay, which was cut off from supplies of Venezuelan oil in 1979 but had been contracted by Coastal to refine part of its Venezuelan shipments.

The principal legal charge brought by the two companies is that Petroleos de Venezuela violated U.S. antitrust laws by ordering its other U.S. clients not to resell supplies of crude oil to Mobile Bay and Coastal.

All Venezuelan contracts with oil companies contain a clause prohibiting resale of oil shipments, a measure Venezuelan officials say is designed to prevent price speculation. The plaintiffs, however, contend that the clause is illegal and that the Venezuelan company further violated the law by specifically instructing its customers not to sell to Mobile Bay in an "orchestrated conspiracy" to drive the company out of business.

According to legal experts, these charges by Coastal and Mobile Bay, if upheld, threaten to invalidate Venezuela's overall marketing system for oil in the United States, along with those of other OPEC suppliers who similarly ban resale of oil shipments.

In addition, however, the U.S. companies charge that this system of market control was laced with corruption. They maintain that some oil companies were allowed by the Venezuelans to resell their oil, including one Texas company that supplied Mobile Bay for several years at a $1 per barrel premium. These companies, the suit charges, paid kickbacks and bribes to the Venezuelans.

Officials of the Venezuelan company have refused to comment publicly on such charges, but instead say they are preparing for a protracted legal battle in U.S. courts.