The embattled president of Argentina, Cristina Fernández de Kirchner, on Friday revoked tax increases on grain exports, a measure that had set off four months of farm revolts, food shortages and nationwide protests.

The Argentine Senate rejected the tax plan early Thursday after a marathon session in which President Kirchner's own vice president, Julio Cobos, broke a tie to defeat the measure.

The move on Friday to revoke it was a humbling retreat by the president and was necessary to stop the hemorrhaging of her seven-month-old presidency, political consultants and risk analysts said. Mrs. Kirchner's government has lost much of its support during the conflict, with her approval rating falling to as low as 20 percent in some polls.

In explaining the decision to rescind the tax plan, Alberto Fernández, the president's chief of staff, said Friday afternoon that in a democracy one had to "respect the popular will" and "preserve the quality of institutions."

He gave no indication of whether the government planned to revisit the measure, but he said it was committed to its overarching vision of redistributing income to the lower classes, a justification the government had used for the tax increases.

Eduardo Buzzi, president of the Argentine Agrarian Federation, told local television reporters that the government's announcement was "a joy for us."

Mrs. Kirchner has said little in reaction to the Senate vote, and has yet to comment directly on the dramatic decision by Mr. Cobos, a former governor from Mendoza Province, to defy his own boss and side with the agricultural producers. Mr. Fernández did not indicate Friday whether the president would request Mr. Cobos's resignation.

On Friday, Mr. Cobos was greeted in Mendoza by throngs of supporters. One had a sign that read "Courage and Honesty." In television interviews Mr. Cobos said he had no plans to resign and had voted his convictions. "There is no one that can ask for my resignation," he said, stressing that "the separation of powers has to be respected." The Senate vote, he added, showed that Argentines "are asking for consensus."

Agustín Rossi, leader of the pro-Kirchner bloc of legislators in the House, said late Thursday that he did not think Mr. Cobos should resign. "I don't agree with his decision, but I respect his right to make it," Mr. Rossi said in a televised interview.

With turmoil mounting in the country and the farmers refusing to back down, Mrs. Kirchner gambled on her seeming control of Congress when she decided last month to send the measure to legislators for approval.

The Senate's failure to support the president and Mr. Cobos's tie-breaking vote stunned the nation, which is accustomed to a Congress that complies with its president's wishes. Just hours before the vote, Mrs. Kirchner's high-ranking officials were seen with supporters at a rally outside the Congress building, anticipating a victory celebration.

Despite the Senate vote, it was up to the president to rescind the tax measure in order for export taxes to revert to the system that was in place before March 11, when she imposed her tax system without consulting Congress.

Under the president's new system, taxes on grains floated up or down with commodity prices, and had reached more than 44 percent for soybeans, Argentina's most important export. The higher taxes were expected to yield more than $3 billion in additional revenue a year for the Argentine treasury. The tax rate will now be fixed at 35 percent.