Senate Democratic leaders backed off yesterday from a threat to hold up Supreme Court nominations to force another vote on judicial nominee Daniel A. Manion, but said they still might delay confirmation of lower-court judges.

Sources said Minority Leader Robert C. Byrd (D-W.Va.) and Sen. Joseph R. Biden Jr. (D-Del.) made clear at a Democratic caucus that it did not make political sense for them to hold up the nominations of William H. Rehnquist as chief justice and Antonin Scalia to fill Rehnquist's Supreme Court seat.

The earlier threat followed statements by Majority Leader Robert J. Dole (R-Kan.) that he may use a rare parliamentary maneuver to confirm Manion without a replay of last month's razor-thin margin of approval -- a vote the Democrats view as tainted.

Biden, floor leader of the fight to defeat Manion's nomination to the 7th U.S. Circuit Court of Appeals in Chicago, said "public pressure" would force the Republicans to allow a second vote. "If Dole pulls that sort of bush-league move, I don't know how he can stand it in terms of his reputation," Biden said.

Nevertheless, Biden said, "I would not like to see us hold Supreme Court justices hostage. They are separate and distinct issues."

The battle over Manion, a conservative South Bend, Ind., lawyer, has been a focus in the debate over the quality of President Reagan's judicial appointments. Reagan repeated his support for Manion during a White House meeting with Republican congressional leaders yesterday.

Dole told reporters after the meeting that the Democrats had a fair shot at defeating Manion last month and should not try to hold up the Supreme Court nominations.

Byrd said yesterday that Manion should not be "slipped into a judgeship . . . through the back door." But he did not endorse a suggestion Monday by Senate Minority Whip Alan Cranston (D-Calif.) that the Democrats consider holding up the Rehnquist and Scalia nominations. Cranston said yesterday that it would be better to consider delaying 38 other pending judicial nominations.

Democrats thought they had the votes to defeat Manion June 26, but wound up losing 48 to 46. Byrd voted with Manion's supporters so he would have standing under Senate rules to move to reconsider the vote.

During the vote, Sen. Slade Gorton (R-Wash.) switched to support Manion after a White House official promised him approval of a district court nominee in Washington state. After time expired, Sen. Nancy Landon Kassebaum (R-Kan.) withdrew her vote against Manion after being told, erroneously, that Sen. Barry Goldwater (R-Ariz.) wanted to vote for Manion but couldn't make it to the floor. Goldwater declined to vote.

Asked about Dole's contention that there is no need for a second vote, Biden said: "It's not like he won. Technically he won, but he won on an intentional or unintentional subterfuge.

"Nancy Kassebaum has said privately and publicly she was misled. We had the vote won," Biden said. "Manion was defeated, 48 to 47. Manion was kept alive by Republican maneuvers that exceeded the agreement we made."

Biden said he assumes that Dole will allow a second vote on Manion this year. But "if Dole plans on running the Senate that way, not allowing a vote, he shouldn't expect comity on other legislation or judicial nominees," Biden said.

Neither side knows whether it has the votes to win another showdown. To defeat Manion, sources say, the 45 Democrats and four Republicans who oppose him must pick up the vote of uncommitted Sen. Bob Packwood (R-Ore.), who did not vote last month, and be assured that Goldwater abstains again. Opponents must do better than a tie, which would be broken in Manion's favor by Vice President Bush.

Biden also said yesterday that Judiciary Committee Chairman Strom Thurmond (R-S.C.) is trying to "steamroller" the nominations of Rehnquist and Scalia by scheduling their confirmation hearings over the next two weeks and not giving the Democrats enough time to prepare. Thurmond has said the Democrats had plenty of notice, but there are negotiations to delay the hearings in exchange for a definite vote next month on both nominees.