WASHINGTON--Sen. Howard H. Baker Jr. (R-Tenn.) yesterday announced he is a candidate for president, although he acknowledged he has not yet demonstrated he can beat his Republican rivals.

"By the time we're in the middle primaries, I've got to stop being in second place and start being in first place," Baker, the Senate minority leader, said yesterday to a group of supporters.

Baker, who joins eight other declared Republican presidential candidates, made his announcement in the Senate Caucus Room, where he attained national recognition six years ago as the senior Republican on the Senate Watergate Committee.

Baker said he would spell out his program for launching "a new generation of confidence" during the upcoming campaign. "Unlike others in the past, I will not deal in promises. I do not ask you to trust me; I ask you to judge me. Promises are cheap, but America needs performance," Baker said.

Baker reaffirmed his protest of the SALT II treaty in his announcement. "Approval of this treaty with its acceptance of Soviet strategic superiority could guarantee to the Soviet Union the margin of error that used to be ours," he said yesterday.