Senate Republicans reacted with enthusiasm and Senate Democrats with caution yesterday to President Reagan's nominations of Justice William H. Rehnquist to be chief justice and Court of Appeals Judge Antonin Scalia to be an associate justice of the Supreme Court.

Initial comments of key senators suggested that the proposed elevation of Rehnquist to chief justice could be the more controversial of the two appointments, with the confirmation battleground centered on his politically conservative views.

But Republicans and Democrats praised the nominees' legal qualifications amid widespread predictions that both will be confirmed by the Senate this year.

Senate Judiciary Committee Chairman Strom Thurmond (R-S.C.), calling for an expedited confirmation process, said he will schedule hearings on the nominations as soon as they are formally submitted by the White House. Republican sources said hearings are not likely to begin until Congress returns from its July 4 holiday recess on July 14, with the length of the hearings dependent on developments.

However, some Democrats and outside groups that may be critical of the nominations urged a cautious approach.

An unusually subdued Sen. Joseph R. Biden Jr. (D-Del.), ranking Democrat on the Judiciary Committee, said he could offer "no opinion" on either nomination immediately.

"I think we should spend some time looking into them thoroughly," Biden said. "I don't think you should take such a significant judgment by the president and reach any immediate conclusions."

The issue of Rehnquist's conservative philosophy was raised by Senate Minority Whip Alan Cranston (D-Calif.), who suggested that Rehnquist is "an ideological extremist" who would not administer the court's business impartially.

"The question is: Can a man who has an extreme right wing ideology manage the court in a fair and balanced way," he said.

Of Scalia, Cranston said, "I gather he is intellectually brilliant and quite conservative." He said he had not made a final judgment on the Rehnquist nomination, but added:

"I'm not talking about a conservative. The court can take a conservative. It can take a liberal. But if you are off to the ideological extremes -- and my impression is that is where Justice Rehnquist is -- then there really are questions about whether you can lead this court and produce decisions that will be accepted by the American people."

Most Democrats, including Sen. Edward M. Kennedy (D-Mass.), the second-ranking Democrat on the Judiciary Committee, said they would "reserve judgment" on both nominations, but Sens. Russell B. Long (D-La.) and William Proxmire (D-Wis.) endorsed Reagan's choices.

"He's a conservative, which is all right," Proxmire said of Rehnquist. "What the hell, everybody's got to be something. But he's extremely well-qualified and very bright intellectually."

Republicans said they doubted an assault on Rehnquist's well-known conservative philosophy would succeed in denying him the position of chief justice. "You need more than ideological differences to give a rough time to two men this eminent," Sen. Orrin G. Hatch (R-Utah), a member of the Judiciary Committee, said. "It would be a disgrace to fight them on ideological purposes."

Sen. Charles McC. Mathias Jr. (R-Md.), the most liberal Republican on the Judiciary Committee, did not endorse either nominee but suggested that Rehnquist's confirmation as chief justice was all but a foregone conclusion. "Justice Rehnquist is well known to the nation and we should not anticipate radical change under his leadership," Mathias said.

However, others in Congress predicted a move to the right by the court as a result of the nominations. Hatch said Scalia is more conservative than retiring Chief Justice Warren Burger and could make a significant difference in close decisions.

On the surface, neither Rehnquist nor Scalia should face major difficulties in winning confirmation. Rehnquist's 1971 appointment to the Supreme Court was confirmed by a then Democratic-controlled Senate by a vote of 68 to 26. Only three current senators -- Cranston, Kennedy and Daniel K. Inouye (D-Hawaii) -- were among those who opposed the nomination.

Scalia's appointment to the U.S. Court of Appeals here in 1982 breezed through the Judiciary Committee and the full Senate with virtually no opposition.

Asked about this, Biden said there are important differences between the Supreme Court and lower courts. While Scalia is "unquestionably qualified to sit on the circuit court and Court of Appeals, it is an open question whether he is qualified to sit on the Supreme Court," Biden said.

In pressing for quick confirmation hearings, Republicans appeared intent on disposing of the nominations well before the fall congressional elections, when control of the Senate, where the GOP now enjoys a 53 to 47 majority, will be at stake. There were no suggestions that Democratic critics will attempt to delay confirmation votes until after the elections, but Cranston said that it "would certainly be an option" depending on the outcome of the Judiciary Committee hearings.

Hatch said delaying tactics "would also be a disgrace," adding: "That won't happen."