Senate Democrats won agreement yesterday for a week's delay in confirmation hearings on Chief Justice-designate William H. Rehnquist and Supreme Court nominee Antonin Scalia, promising in return that the full Senate will take up both nominations in early September.

The agreement minimizes the possibility of a long, drawn-out battle over Rehnquist and Scalia, who are expected to win confirmation easily. But the Democrats were eager to buy time because they want to dissect each judge's record and leave open the possibility that revelations might surface before the final vote.

The arrangement, negotiated by Judiciary Committee Chairman Strom Thurmond (R-S.C.) and ranking Democrat Joseph R. Biden Jr. (Del.), gives the Democrats more time to prepare to question Rehnquist, whose confirmation hearings were delayed until July 29 and July 30. The hearings on Scalia, a federal appeals court judge here, were set for Aug. 5 and Aug. 6.

Thurmond had previously scheduled Rehnquist's hearing for next Tuesday, sparking complaints that the Democrats did not have enough time to study Rehnquist's 15-year record as a Supreme Court associate justice. Under the agreement, the Judiciary Committee will vote on both men Aug. 14 and the full Senate will consider the nominations in the first week after its summer recess.

The written agreement -- which was endorsed by Majority Leader Robert J. Dole (R-Kan.) and Attorney General Edwin Meese III -- contains "a caveat that if some substantial, unanticipated fact arises, additional time will be negotiated." The Democrats also promised not to block a motion to proceed on the nominations, although they can filibuster once debate begins.

While not part of yesterday's agreement, Senate sources say there is also an understanding that Dole will allow a second vote next week on the hotly contested appeals court nomination of Daniel A. Manion of Indiana. The Senate approved Manion last month in a vote marked by charges of irregularities, and Dole had said he might invoke a rare parliamentary maneuver that would result in Manion's confirmation without another vote.

Sources said Manion's Republican supporters now think that the opposition has come up one vote short. Forty-five Democrats and five Republicans are on record as opposing Manion, and a 50-to-50 tie would be broken in Manion's favor by Vice President Bush.

One Republican who voted against Manion, Sen. Daniel J. Evans (Wash.), now says through a spokesman that he is "leaving his options open" and "leaning toward" opposing a procedural motion for a second vote, on grounds that Manion's critics "had their chance to vote." Evans' home-state colleague, Sen. Slade Gorton (R-Wash.), touched off a storm of criticism when he switched his vote in Manion's favor in exchange for White House approval for a district court nominee in his state.

While the Democrats have attacked the South Bend, Ind., lawyer as too inexperienced for the federal bench, they face a political dilemma in the hearings on Rehnquist and Scalia, whose aggressive and articulate conservatism is matched by their legal credentials.

As if to underscore the point, an administration source disclosed yesterday that the American Bar Association's screening committee has found Rehnquist and Scalia "well qualified," its highest rating for the Supreme Court.

Biden has assembled a team of legal experts -- Washington attorney Kenneth Bass and law professors Laurence Tribe of Harvard, Walter Dellinger of Duke and Philip Kurland of the University of Chicago -- to pore over thousands of Rehnquist opinions and help the Democrats prepare for the hearings.

Still, one Senate Republican aide said, "The Democrats cannot conduct an ideological battle because they've said all along their issue is quality. Their hands are tied."

The Democrats have not completely renounced ideology as a basis for challenging judicial nominees. In a speech last November, Biden said a president must select judges "from the mainstream of American jurisprudence" and that some candidates may be too extreme to pass Senate muster.

A Senate Democratic aide said Biden's staff wants to evaluate Rehnquist's opinions in civil rights and First Amendment cases, as well as the 54 "Lone Ranger" opinions in which Rehnquist was the sole dissenter. "Are some of the things he's been writing on the outer fringes of American legal thought," the aide asked, "or are they just at the edge of the conservative mainstream?"

Dellinger said he and other legal experts, at a 3 1/2-hour session with Biden this week, discussed whether Rehnquist's "ideological views are so strong that they swamp any analysis of particular cases . . . the degree to which he has been consistently insensitive to civil rights claims by minority groups . . . whether he is beyond the pale of acceptable judicial philosophy."