Senate Majority Leader Robert C. Byrd urged President Carter yesterday to reject the bid from Saudi Arabia for additional military equipment for the F15 fighter jets it is buying from the United States.

Stressing his strong support for Saudi security needs, the West Virginia Democrat defended the F15 sale, arranged two years ago, but said the new requests involve offensive weaponry.

"I see no rationale that would justify this kind of offensive equipment," Byrd said."I would hope the administration does not pursue it."

The Carter administration has not decided what to do about the Saudi request, which reportedly involves additional bomb racks, fuel tanks and missiles for the F15s. Congress specifically prohibited any supplementary equipment sales in approving the F15 deal in 1978.

The sale of new tanks to Jordan, announced this week during King Hussein's visit to Washington, is a different matter because similar tanks are being supplied to Israel as well as Saudi Arabia, Byrd said.

The majority leader told his regular Saturday news conference that President Carter "made a mistake" in rebuking West German Chancellor Helmut Schmidt for Schmidt's go-slow position on nuclear weapons deployment in Europe.

Unity among the western allies is important now Byrd said, and Carter should have no reason to oppose talks on limiting arms at a time allied forces continue modernizing. "I'm under the impression that Schmidt is committed to that," Byrd said. He added he hoped deployment of medium-range nuclear missiles in Europe would proceed as scheduled.

On domestic matters, Byrd said any effort by the Democrats to keep Rep. John Anderson (R-Ill.) off state ballots in November could backfire on President Carter and hurt other Democrats.

The attempt "could make a martyr out of [Anderson,]" Byrd said. "I think the Democratic National Committee could better-spend its money helping Democratic senators and congressmen get elected."

The DNC has denied reports it plans to launch a major drive to keep Anderson off as many state ballots as possible to prevent him from drawing votes from Carter. Byrd noted that the Republican National Committee has $5.5 million to spend on Senate races, compared to $800,000 at the DNC.

"The DNC doesn't exist just to help the president," Byrd said.

He added that "chances are looking up" for the 24 Senate Democrats facing reelection to the point where "there's a possibility of a net gain" among the 10 GOP seats also at stake.

The reasons, he said, include intensive Democratic campaigning and "Madison Avenue soap opera advertisements" by the Republicans. GOPpresidential candidate Ronald Reagan is ignoring "crushing reality" in what Byrd called "naive, jingoistic" foreign policy statements, Byrd added.

Senate Minority Leader Howard H. Baker Jr. (R-Tenn.) will have to wait awhile before taking over the majority leader's job, Byrd joked. "Although Mr. Baker is constantly measurring my windows for curtains . . . I will still be selecting the curtains for this office," he said.