White House Chief of Staff James A. Baker III conceded yesterday that the turnaround in the economy forecast for this year by the administration "probably will not be as strong as we originally hoped."

Baker suggested that the administration might review the economic recovery program after the fall elections, but he said President Reagan is convinced the current plan eventually will work.

"Will it work in time to benefit you politically?" Baker asked rhetorically. "That I concede is a question."

The low-key and precise Texan, who has emerged to become the preeminent of Reagan's "Big Three" domestic policy advisers, reflected the new politics of lowered expectations as he answered questions during a luncheon session with reporters.

Some of his points on administration expectations of the economy's performance and concern about losses in the fall elections had been expressed by administration officials traveling with Reagan in California last week.

Baker said he thought the Republicans would hold on to the Senate ("absolutely") and he thought they would be able to limit losses in the House. But he acknowledged that there had been some erosion in support for Reagan since he took office. He said that is because Reagan is an "activist president" and may have offended some by his cuts in federal spending.

The recession has bottomed out, Baker said, and he contended that there are "some signs" that the economy was moving upward. It is "still too early to tell," however, whether the July 1 tax cut will stimulate consumption and spur recovery, as the administration had once forecast, he added.

Saying it was not unusual for administrations to review economic and foreign policy after midterm elections, he indicated that the Reagan White House would probably do so also, but he said there was no existing economic "fallback" plan.

"Our fallback plan is to continue working as hard as we know how to make the existing economic program work," he said.

Baker said the top legislative priorities of the White House are getting action on a budget and tax bill, Reagan's Caribbean Basin Initiative, enterprise zones, crime package and tuition tax credit proposal and the balanced budget amendment.

He also suggested that Reagan may decide to call Congress back for a lame duck session after the fall elections if there is still work needed to be done. Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) has announced an Oct. 2 adjournment to give members time to campaign for reelection.

It's "too early to judge" whether there will be a need for a lame duck session, James Baker said, but "if there are some things that can be productively done, then we would like to see a special session."

Responding to questions about the persistent rumors that he might like a post in the Cabinet after the election, Baker would only say that he would serve wherever Reagan wants him to serve. Asked if he entertained ambitions to be president himself, he said, "I'm not so interested."