Senate Minority Leader Robert C. Byrd (D-W.Va) sharply criticized Secretary of State Alexander M. Haig Jr. and Defense Secretary Caspar W. Weinberger yesterday, saying they exhibited "bad judgment" in leaving the country "when mmany signs point to an iminent Soviet invasion of Poland."

With the Polish crisis reaching its "most crucial stage" and President Reagan hospitalized with a bullet wound, Byrd said, both secretaries should have postponed trips abroad. Haig left ona 10-day official visit to the Mideast Friday and Weinberger is on a week-long trip to Europe.

"I feel it is a mistake and certainly a case of bad judgment," the Democratic Senate leader said at his customary Saturday news conference. "It sends a bad signal to the Soviet Union. It sends a bad signal to our allies and it leaves the impression the administration still isn't very well organized in its policy toward such crisis."

The trips by the two leading Cabinet members, he said, also leave the impression that the U.S. government is not concerned about the Polish situation, and come at a time when Reagan, recuperating, "needs the advice and counsel of the secretary of state in particular."

State and Defense Department officials have expressed increasing concern in recent days over the Soviet troop activity around the Polish border and have warned about the possibility of an invasion. But spokesmen scoffed at suggestions that the two secretaries should have delayed their long-scheduled trips.

Byrd said he sees no possibility for U.S. military intervention if a Soviet invasion takes place, but indicated economic and diplomatic pressures would be appropriate.

The minority leader said his criticism of Haig was not related to criticism some directed at the secretary last week for his actions in the hours after Reagan's shooting, when Haig appeared before television cameras and declared he was in control of the government.

The West Virginia Democrat took special care to praise Haig as a man of "considerable talents," and said the Reagan administration handled events around the assassination attempt as well as could be expected.

But Byrd said the incident raised questions about "holes" in the Constitution's 25th Amendment, which deals with presidential succession. He expressed particular concern that the amendment does not set out an automatic procedure whereby the vice president could become acting president when the president is temporarily unable to perform his duties, such as while Reagan was undergoing surgery Monday.

Under the amendment, the vice president becomes acting president only if he and a majority of the Cabinet agree to such an action or if the president submits a written declaration to the Senate that he is unable to perform his duties.

Under this system, Byrd said, any vice president who asked the Cabinet to name him acting president would create a panic and be accused of trying to seize power even if the president were temporarily unconscious and unable to make needed emergency decisions.

Byrd also said he intends to introduce legislation requiring mandatory life imprisonment for any assassination attempt on a president, a president-elect or a member of Congress. Currently, only successful assassinations carry a mandatory life sentence.

Byrd said assassination attempts "should be lifted out and above ordinary murder attempts" because they can affect "the whole government system."