Democrats pressed the case yesterday for a limit on U.S. tanker-escort operations in the Persian Gulf, with Sen. Dale Bumpers (D-Ark.) calling on President Reagan to "obey the law" and invoke the War Powers Resolution.

Bumpers, in the Democatic response to the president's weekly radio broadcast, said that if Reagan intends to keep U.S. naval forces on the escort operation until the end of the Iran-Iraq war, "we're at the mercy of Iran." There is also the possibility of American forces "losing their lives to American-made weapons" that were supplied to the shah of Iran before his downfall or were sold to Iran by the administration, he said.

Bumpers' statement came as the Senate prepares for a crucial vote next week on a proposal to require congressional approval for continuation of the escort operation for more than 90 days after enactment of the legislation.

The plan was drafted by Senate Majority Leader Robert C. Byrd (D-W.Va.) and is expected to come up for a vote Wednesday as an amendment to the $302 billion Defense Department authorization bill.

The Senate met yesterday and debated the bill but did not take any roll call votes on it.

Reagan denounced the Byrd amendment Friday and threatened to veto the bill if the Byrd amendment is attached, lest it eventually force U.S. military forces out of the region. However, the Byrd amendment specifically targets only the reflagging and escorting of Kuwaiti tankers. Bumpers said there is no intention to force an end to the U.S. presence in the area.

Under the War Powers Resolution, passed in 1973, whenever the president commits U.S. forces to hostilities or imminent danger of hostilities without congressional approval, he must notify Congress, which then has 60 days to approve or disapprove.

Bumpers said the purpose of the act is to prevent unilateral action by a president that "could lead the U.S. into an accidental war" without the approval of Congress. He said that is precisely the danger in the current Persian Gulf action, which included a recent U.S. attack on an Iranian vessel that was laying mines, resulting in the death of three Iranian sailors.

"It is the law of the land. And the president has no right to violate the law," declared Bumpers, adding, "We're pleading with you. Obey the law."

Successive administrations have criticized the War Powers Resolution, arguing that it can tie a president's hands or send a signal to other countries that Congress may block further U.S. military action. Reagan has refused to send Congress the notice required by the act to start the 60-day period for congressional action. The Senate last week defeated, 50 to 41, a plan by Sens. Mark O. Hatfield (R-Ore.) and Lowell P. Weicker Jr. (R-Conn.) to trigger the act.

The Byrd amendment, if enacted, would substitute for the War Powers Resolution in the dispute over the Persian Gulf escort action, but the amendment would be void if the president vetoes the defense bill and Congress does not override the veto. Even without the tanker-escort amendment, the bill faces a possible veto because of restrictions on his "Star Wars" antimissile program, known as the Strategic Defense Initiative.