The sense of strain is rising in Mexico, and the crisis that began with its foreign debts four years ago is becoming sharper. The dismissal of the Mexican government's chief debt strategist, Finance Minister Jesus Silva Herzog, seems clearly to be related to the divisions within the government over its next move. The drop in the price of oil since January represents a loss to Mexico of one-fourth of its federal revenue and one-third of its export earnings. The enormous budget deficit is pushing up the inflation rate, and capital flight has begun again. In the midst of these tensions, the Mexican government is being subjected to Sen. Jesse Helms's abusive and inflammatory hearings here in Washington.

It's easy for Americans, who know Sen. Helms well, to brush off his attacks on Mexico. But it's harder for Mexicans. There are fears in Mexico that the United States is trying to do to President Miguel de la Madrid what it did last year to President Ferdinand Marcos of the Philippines -- to undercut his reputation and authority in preparation for a change of government. That suggestion is considered ludicrous here. But to many Mexicans, accustomed to their own tightly centralized politics, it is inconceivable that a senator of President Reagan's party could launch hearings of this nature except at the administration's direction.

Last month Sen. Helms took testimony -- much of it evidently inaccurate -- on corruption and drugs. This week he charged massive fraud in Mexico's last president election, a direct assault on President de la Madrid's legitimacy. If the administration does not intend to let Sen. Helms take over its Latin American policy, President Reagan is going to have to step forward, personally and publicly, to reassure Mexico that the senator speaks for no one but himself.

Why is Sen. Helms doing it? To punish Mexico for its refusal to support the United States in Nicaragua is some part of it. Perhaps he also hopes that a change of government in Mexico would bring the far right to power. That is a grievous misconception of Mexican politics, for nationalism there pulls to the left.

But it would be an equally serious mistake on the part of the administration to underestimate the degree to which the Helms hearings can poison the atmosphere in which the debt negotiations go forward. For Mexico and its friends, dealing with the debts this year is both easier and harder than in 1982, when it suddenly declared its inability to repay. It is easier because Mexico is now an isolated case. The falling oil prices that have caused Mexico's distress promise to benefit most of the other Latin economies. But dealing with the debts is harder because Mexico is wearying of the long years of negotiations and the feeling that it's at the mercy of the foreign bankers. There's a rising temptation to indulge in a sweeping gesture such as a unilateral moratorium on payments, despite the knowledge of the great economic damage that would follow.

To keep the talks on a rational and productive level will require great strength on the part of President de la Madrid and his government. It may also require a more visible demonstration of American support than the administration has yet provided.