The Reagan administration plans to force Democratic members of the U.S. Parole Commission to step down before they finish their six-year terms so they can be replaced with Republicans.

Commissioner Oliver J. Keller, who has 2 1/2 years remaining in his term, was the first to get formal notice that the administration will try to force him out. He says he was called Dec. 16 and given the choice of either resigning by Jan. 15 or being transferred to Kansas City where he would be expected to resign by March 15.

But Keller said lawyers for the commission believe the move is illegal and he has decided not to cooperate. "I don't plan to resign at all. I think it's rotten politics. It's the spoils system at its worst."

"There has never been an administration before that has tried to do this," Keller said. " The Parole Commission has always been considered something that should be outside the spoils system. We decide who gets out of jail. That should not be a political decision."

The Senate has already confirmed Benjamin Baer, President Reagan's choice to replace Keller.

Associate Deputy Attorney General Stanley E. Morris confirmed yesterday that there are plans to replace the six Democrats on the nine-member commission. He said the main political objective would be to find Republicans with a "stronger law enforcement bias" than present members have. He said Reagan's appointees are likely to be "less lenient" in their parole policies.

"It's quite simple," he said. "There was an election a year ago . . . . Our view is that a Republican president ought to have the right to appoint Republican people to executive positions."

Asked if it is legal to dismiss the Democrats from the commission even though they have been confirmed by the Senate for six-year terms, Morris said, "You're asking the Justice Department if you can do that legally? Of course it's legal."

Members of the House Judiciary Committee say that no other administration has attempted to politicize the parole commission.

Rep. Robert W. Kastenmeier (D-Wis.), chairman of the subcommittee on courts, civil liberties and the administration of justice, which has jurisdiction over the parole commission, said the administration "will not be permitted to get away with this outrageous, brazen move."

The Judiciary Committee sent a letter last week to Attorney General William French Smith protesting "any suggestion that the independence of this important agency should be subject to the partisan whim of this administration or any other."

"The notion that a parole commissioner may be removed for mere partisan purposes is not consistent with the law . The present situation, we understand, does not involve any allegation of wrong-doing on the part of a commissioner but rather has a political purpose," said the letter, which was signed by Chairman Peter W. Rodino Jr. (D-N.J.) and the chairmen of the Judiciary subcommittees.

The Parole Commission is an independent agency within the Justice Department with the power to deny or approve parole applications, issue regulations and create regional offices. The commissioners' jobs are full-time.

Kastenmeier said, "The law specifically provides for a fixed, six-year term for each parole commissioner, appointed with the advice and consent of the Senate. They do not serve at the whim of the president or attorney general." He said the precedent set by the political removal of a Senate-confirmed commissioner would be "devastating."

"The Federal Trade Commission, the Federal Communications Commission, the Securities and Exchange Commission and all other independent agencies could easily become political toys for the president if this . . . is not challenged," Kastenmeier said. He warned that if the administration goes forward with the plan it will face full congressional oversight hearings and a court fight.