A Senate Judiciary subcommittee at the request of the Reagan administration yesterday unexpectedly postponed its scheduled opening hearing today on an extension of the 1965 Voting Rights Act.

A spokesman for the subcommittee on the Constitution, whose chairman is Sen. Orrin G. Hatch (R-Utah), said the White House had requested the postponement "to accommodate efforts by the Reagan administration to develop proposed legislation on this subject."

There was speculation that the White House, under attack because of its civil rights policies, might be about to alter its position on voting rights.

But an administration source said the delay was requested for tactical reasons by Attorney General William French Smith and that there is no plan to modify the administration position.

He said Smith, who was scheduled to testify today, felt his testimony would have more impact after Congress returns from its recess next week and the members are all here to hear or read about it.

Ralph Neas of the Leadership Conference on Civil Rights immediately criticized the postponement, saying, "Any delay can only be seen as an effort to undermine the voting protection of minority citizens.

"The Reagan administration plea for additional time in which to submit legislation is no excuse for delaying hearings which are starting a bare seven months before the scheduled expiration of the act. A year has already elapsed in which the administration has failed to submit an act," he said.

Last year the House voted 389 to 24 for a strengthened extension of the Voting Rights Act, and an identical bill has been introduced in the Senate by 61 cosponsors, enough not only to guarantee passage but also to override a filibuster.

The White House has said it would prefer a simple extension of the existing law.

The hearings have been tentatively put off to Jan. 27.