White House chief of staff Howard H. Baker Jr. said yesterday that President Reagan will move quickly to find a replacement for Supreme Court Justice Lewis F. Powell Jr., and administration officials emphasized that "confirmability" will be a major requirement.

Senior White House officials said Powell's resignation provided what one called "both an opportunity and a challenge." He said the resignation of Powell, a frequent swing voter, gives Reagan an opportunity to make the high court more conservative and leave a legacy that will last for many years.

"The challenge is to find a candidate who is highly qualified, reflects the president's philosophy and can be confirmed by a Democratic Senate," the official added.

One senior official also said that "regionalism" is likely to play a role, pointing out that Powell was "the only southerner on the court" and saying that this point is likely to be considered in White House discussions.

Baker yesterday began the process for finding a replacement soon after he told Reagan that Powell had resigned. Baker directed White House counsel A.B. Culvahouse Jr. to prepare a list of potential candidates based on material in the files.

"It's not a big list," added the White House chief of staff, who ruled out himself as a possible nominee.

A senior official said the nomination could come as early as next week. White House spokesman Marlin Fitzwater, after conferring with Baker, said, "We will search and have a nominee very soon."

These officials say it is essential for Reagan to move quickly so that a nominee can be approved this year. They said confirmation of any major appointment, especially for the Supreme Court, is likely to become far more difficult in 1988, an election year and the last of Reagan's presidency.

Baker said that a discussion on the nomination would be held Monday with the president and Attorney General Edwin Meese III, who was en route from West Germany when Powell announced his resignation. Meese has played a major role in the selection of Reagan's prior nominations to the high court.

White House officials had an inkling late Thursday that there would be a resignation when Chief Justice William H. Rehnquist's secretary called Baker's secretary to tell him that Rehnquist would be calling the following morning.

Baker was holding his regular morning meeting with the president when Rehnquist telephoned to tell him that Powell was resigning. The chief of staff took the call in a side room, then returned to the Oval Office to inform Reagan.

"The president acknowledged it {and} expressed . . . mild surprise, not great surprise," Baker said.

Powell's resignation has the potential for rekindling a struggle within the administration between conservative and moderates that has been largely dormant as Reagan has struggled to regain his political equilibrium amid the Iran-contra scandal. But one senior official said that "even staunch conservatives recognize that the president has been damaged and will have to show some practicality about his nomination."

However, Sen. Gordon J. Humphrey (R-N.H.), a frequent promoter of conservative causes, said that Powell's resignation gives Reagan opportunity to reverse the "infamous Roe v. Wade decision which legalized abortion." Powell voted with the majority in this 1973 case.

Beyond that, Humphrey said, the vacancy provides "a great opportunity for the president to remold the court along more conservative lines."

This hope was the fear of Sen. Edward M. Kennedy (D-Mass.), who said the Senate will be watching carefully "to ensure that President Reagan does the right thing instead of the far-right thing in filling the large vacancy that Justice Powell leaves."

Reagan issued a statement praising Powell as a "wise and generous influence" who would be missed. He was quoted by Fitzwater as telling Powell in a phone call, "The country owes you a great debt."

The "confirmability" yardstick raised yesterday by White House officials appears to rule out the selection of Meese, who underwent a bruising fight to be confirmed as attorney general. Baker declined to speculate on whether Meese is interested in the job.

Other Californians who have been members of the Reagan inner circle, notably former attorney general William French Smith and former secretary of the interior William P. Clark, who was a Reagan appointee to the California Supreme Court, have said they are not interested in an appointment.

An official close to the selection process said that such potential nominees as Sen. Orrin Hatch (R-Utah) and Robert H. Bork, a federal appeals court judge in Washington, are "obvious possibilities" but added that "we're going to look very closely at the pros and cons of both qualifications and confirmability" before making any recommendation to the president.